Thanks! That was stupid of me, I even have a view like that set up for exactly that purpose. Though I hadn't included the 30 day SEC yield column before.
Definitions matter. If you look at the M* SPAXX page I linked to, under the "Performance" tab you'll see a one month return of 0.36%.
Now look at the "Chart" tab. Click on the 1M setting. A different gain - 0.37% (actually 0.3727%). At least there we can see that this return is from 9/24/24 through 10/25/24.

Maybe one is fWAR and the other is bWAR. :)

Thanks! That was stupid of me, I even have a view like that set up for exactly that purpose. Though I hadn't included the 30 day SEC yield column before.
Definitions matter. If you look at the M* SPAXX page I linked to, under the "Performance" tab you'll see a one month return of 0.36%.
Now look at the "Chart" tab. Click on the 1M setting. A different gain - 0.37% (actually 0.3727%). At least there we can see that this return is from 9/24/24 through 10/25/24.
30 days hath September, April, June, and November ... or something like that.
https://poets.org/poem/leap-year-poem
And it gets worse. Here's another Fidelity page giving 1, 7, and 30 day returns for SPAXX, without saying what exactly they represent. My best guess is the average daily dividend payout over the past 1, 7, and 30 days annualized (i.e. no compounding). But who knows?
https://institutional.fidelity.com/app/fund/sasid/details/458.html

You are correct. In the way that matters - what buttons you press. You do just place a single order to buy a Vanguard fund with outside money. Though the verbiage I'm reading seems to suggest that under the covers Vanguard is automatically "laundering" transferred money through the settlement account. Admittedly a distinction without a difference. Just confusing.
On the webpage describing the transition from legacy to brokerage platform are descriptions of how each works. Of legacy platform purchases, it says "Money coming in from a bank or other financial institution directly purchased shares of the mutual fund."
In contrast, for brokerage platform purchases it says:

Your brokerage account comes with a settlement fund that's used to pay for investments and hold assets from investment sales and other transactions. Money from bank transfers or redemptions remains in the settlement fund until you use it to purchase investments or transfer it out of your account.

Emphasis in original. The contrast between the two descriptions (legacy and brokerage platforms) seems to be saying that all money used for transactions comes out of the settlement account, even if it sits there for only an instant ("until you use it").
I tried a test purchase in a zeroed out Vanguard account. I was informed:
You don't have enough money in your settlement fund to cover this order. Transfer additional funds from your bank to proceed with this order.
But I could still place the fund purchase order:

If you wish to use your settlement fund to pay for this order, select the amount needed to cover funding option. If you’d like to maintain your settlement fund balance, select transfer full order amount, or select fund order later.

The first option is to pay for the purchase from the settlement account. The middle option is to "transfer [the] full order amount". This is where Vanguard may be moving money into the settlement account and automatically, instantaneously applying it from there to the fund purchase.
The last option, "fund order later", refers to the ability at Vanguard to purchase securities without having enough cash presently in the account. You do have to cover the purchase by the settlement date. See other MFO thread discussing such transactions.
I think the timing in the Vanguard message below discussing that last option is wrong as settlements are now T+1.

Important, please read
This purchase exceeds the funds available balance in your account’s settlement fund. You must transfer money into your settlement fund within 2 business days. Otherwise, securities in the account may be sold to pay for the purchase, and the account may be restricted from further trading.

If M* can be believed the SEC yield for SPAXX is 4.31 to 4.29 for RPHIX though the total return for the latter is still slightly ahead.

I'm not sure where you found any SEC yield for SPAXX on M*. Here's M*'s page for the fund: https://www.morningstar.com/funds/xnas/spaxx/quote
Likewise, I can't find the 30 day SEC yield for RPHIX on M*.

I'm reporting what I see in the watch list I've created at M*. To my custom view I can add %SEC Yield, and 4 Week Total Return which gets relabeled to Total Return 1 Month on the view. M* then fills those fields with numbers. Where those numbers come from I couldn't tell you. Your work is an example of why I prefaced my post with: "If M* can be believed."

Well, at Vanguard Brokerage, when one uses Buy, there are options to use the settlement fund VMFXX OR to transfer money from linked bank account. The latter works just as well as the old mutual fund purchases.
What is VG doing with its mutual-funds-only 401k/403b? Will those accounts suddenly become brokerage a/c or brokerage-windows within 401k/403b?

And 7-day for SUTXX at Schwab is 4.68%, YTD = 4.08%, also as of 10-25-24.

That's about the same as FSIXX (4.69%), available at Fidelity w/$1M min, or at Merrill w/$1 (sic) min.
https://olui2.fs.ml.com/Publish/Content/application/pdf/GWMOL/ICCRateSheet.pdf
I still expect RPHIX to outperform after-tax, but it will be close. As far as MMFs go, especially in high tax states, these Treasury-only MMFs seem like no-brainers.
https://fundresearch.fidelity.com/fund-screener/results/compare/short-term-perf/averageAnnualReturnsYear3/desc/1?order=&tickers=RPHIX,FSIXX

If M* can be believed the SEC yield for SPAXX is 4.31 to 4.29 for RPHIX though the total return for the latter is still slightly ahead.

30 day and 7 day SEC yields measure different things.
The yield for MMFs including SPAXX is the average current (i.e. distribution) yield (before compounding). The yield for other funds is a 30 day average yield approximating YTM, i.e. projected future total return.
To simplify the non-MMF case, consider an OEF holding a single bond to maturity. If that bond matures in one year, pays a 7% (of face value) coupon semiannually, and is priced at $1,020. Then its YTM (SEC yield) is 4.902%. Its distribution yield is 7% x $1000/$1020 = 6.863%.
http://www.moneychimp.com/calculator/bond_yield_calculator.htm
SPAXX's 7 day yield as of 10/25/24 is 4.49%, compounding to an annual yield of 4.59%.
https://fundresearch.fidelity.com/mutual-funds/performance-and-risk/31617H102
I'm not sure where you found any SEC yield for SPAXX on M*. Here's M*'s page for the fund: https://www.morningstar.com/funds/xnas/spaxx/quote
Likewise, I can't find the 30 day SEC yield for RPHIX on M*. Nor can I find it on RiverPark's site. The best I can find there is an SEC yield of 4.61% as of June 30th. While I do find an SEC yield of 4.29% at Schwab.
However, going by that source, Schwab also gives the current (7 day SEC) yield of SPAXX as 4.49%. Same as Fidelity (to be expected).
My bottom line: RPHIX has outperformed SPAXX over the past 1 month through 9/30, I believe, which I'll take as approximating current performance. In October (through 10/25) RPHIX has returned 0.31% (per M* graph). Without compounding, that's 4.5%, still slightly outperforming SPAXX.
https://fundresearch.fidelity.com/fund-screener/results/compare/short-term-perf/averageAnnualReturnsYear3/desc/1?order=&tickers=RPHIX,SPAXX

And 7-day for SUTXX at Schwab is 4.68%, YTD = 4.08%, also as of 10-25-24.

Hi @WABAC
The current 7 day yield for the Fido MM, SPAXX = 4.49%. Down from last weeks 4.52% yield.
YTD return is indicated at 4.10%, as of 10-25-24

Here is another article:
https://www.thinkadvisor.com/2024/10/25/vanguard-to-close-legacy-mutual-fund-platform-by-end-of-2025/

As mentioned by @Catch22 in this thread, the yield on the 10 year treasury has increased 17.4% since September 16.
Under the circumstances, what have you done for me lately means between the last week and the last four weeks (YMMV.) Over that time period some very nice short term funds are under-performing SPAXX. If M* can be believed the SEC yield for SPAXX is 4.31 to 4.29 for RPHIX though the total return for the latter is still slightly ahead.

Well. I won't argue with the numbers, then. Yes, my two junk funds are TUHYX and PRCPX. The former is two and a half times bigger than the latter. And I also hold Investment Grade WCPNX. I got into my junk just exactly at the wrong time, before the Fed's rate hiking cycle. I rode the buggers down, then back up. Reinvested all profit. I'm ahead of the game by now. Since 2018, there have been 2 terrible years, one of them being the historically putrid year of 2022. And PRCPX: right now having its worst year since 2015.
Funny, how Morningstar's performance charts don't look like they match the actual sadistics. But it's Morningstar, so... I've looked and looked for similar yields elsewhere. I'm not married to these two, but I want to discover something else that looks better to me, before I make a move. If The Fed reduces rates again, I suppose these two will benefit, like all the others.

@Crash, I know you are a big proponent on yield. Myself, it's all about total return. And, I'm a graphic trend person. Here is some total return data for 5 of the funds mentioned here. I'll give total accumulative return starting from 2021, 22, 23 and ytd 24.
from 2021 to present:
DHEAX +14.9%
WCPNX +1.7
RPHIX +16.5%
CSOAX 21.2%
BBBMX 12.8
2022 to present
DHEAX +12.5%
WCPNX +0.3%
RPHIX +14.2%
CSOAX 15.2%
BBBMX 11.7%
2023 to present
DHEAX +16.4%
WCPNX +10.3%
RPHIX +10.t%
CSOAX 22.1%
BBBMX 12.6%
2024 to present
DHEAX +7.5%
WCPNX +3.4%
RPHIX +4.7%
CSOAX 7.4%
BBBMX 5.3%
I'm certainly not a bond expert, nor do I understand categories and duration very well, but I do understand total return as viewed by graphics. I guess my only point here is a fund's total return is more important that a funds yield. You will also see in the picture in the link below which funds had more volatility getting to the end results.
https://stockcharts.com/freecharts/perf.php?DHEAX,WCPNX,RPHIX,CSOIX,BBBMX

@Crash - you don't name your two funds here. But the only taxable bond fund with a 2.49 year effective duration (per M* screener) is TUHYX. The only bond funds with a 2.68 effective duration are PRCPX, RNOTX, and MHCAX.
Ultra low volatility? Five year figures are:
RPHIX: 1.00
FLRN: 1.98
WEFIX: 2.47
FPNRX: 2.52
BBBMX: 2.56
MHCAX: 8.25
RNOTX: 8.28
PRCPX: 9.28
TUHYX: 10.41
Max drawdowns? Again over five years:
RPHIX: 1.09%
FLRN: 3.31%
WEFIX: 4.34%
FPNRX: 4.24%
BBBMX: 4.07%
MHCAX: 12.62%
RNOTX: 13.58%
PRCPX: 13.98%
TUHYX: 17.60%
How about churn? Current turnover figures are:
MHCAX: 20%
BBBMX: 22%
PRCPX: 36.20%
WEFIX: 37%
FLRN: 40%
FPNRX: 50%
RNOTX: 75%
TUHYX: 87%
RPHIX: 372%
BBBMX looks better and better.

COALX. @Bee got me good. I actually looked it up. :)
Here are funds I track that returned between 8.7 and 7.23 over the last 12 months while having a positive return in 2022. They are CBLDX, CBRDX, JAAA, FLTR, NEAR, and VRIG.
If you were willing to experience a drop <= 1% in 2022, you could have made between 10.71 and 7.31 with the following funds: DDFLX, FFRHX, TBUX/TRBUX, and VNLA.
The reason I include 2022 is because I see no reason to assume that rates will fall predictably; although it's hard to see them rising as far from where they are now as they rose in 2022.
There may be lots of other funds out there that I missed.

https://www.hawaiinewsnow.com/2024/10/25/dozens-hawaiian-airlines-employees-separate-with-company-following-merger/
(Irritatingly loud start on the vid with the fabulous news announcer SCREAMING the news.)

@msf In your scenario it makes a bit more sense, but in my case it was more like I had a limit for 200 shares @ $50, got filled for 100, and the next trade came in for 50 shares @ $49.90. So my standing order could have absorbed the entire block with some room + the price difference was a little bit more appreciable...

It's not hard to construct sequences that could account for this. For example, suppose the security is trading at $50.10 when someone enters a limit order to buy 50 shares @ $49.90. Then you enter your limit order to buy 200@50. No sell orders at market price come in.
The next sell order entered is a limit order of 100@$50. Yours is the only open buy order that will meet that price, so you get the 100 shares. You have a remaining order to buy 100@$50.
The next sell entered is a limit order of 50@$49.90. Perhaps the seller entered a price lower than what you were offering because they were afraid that your offer would be snatched by some other seller swooping in and wanted to make sure their sale went through. Perhaps the seller didn't look at the depth of book and just extrapolated the sale price ($50.10, then $50.00, then $40.90). In the end, the actual reason why the seller placed the limit order at $49.90 doesn't matter.
There are two open buy orders that could purchase those 50 shares at $49.90. First one (not you) wins.

I listened to a very tiny little bit of it. They were talking about how the 2014 predictions said small cap were going to do better than large cap over the next 10 years LOL. I then closed it.

backtesting is so often comical

"It depends on what types of risks you're concerned with and how long you're willing to wait for recoveries."
+1
Just an observation -
Is it possible that DHEAX has more unrated stuff that can act weird in a quick flight to safety situation? Not suggesting it is otherwise the same risky as the other two. I would not be surprised if DHEAX overall credit rating is lower.
DHEAX has marginally more securitized stuff than FPNIX now.
I did not know about BBBMX and it looks very good (A+ per M*). I personally am inclined not to buy AA or above stuff (FPNIX) because I do not want to over insure. If I am skipping single A, then I tend to go straight to Treasuries or Agencies. MM are a general exception.
Looking at the chart from DHEAX inception also gets my vote for BBBMX.

@msf In your scenario it makes a bit more sense, but in my case it was more like I had a limit for 200 shares @ $50, got filled for 100, and the next trade came in for 50 shares @ $49.90. So my standing order could have absorbed the entire block with some room + the price difference was a little bit more appreciable... Would you still feel the same about the hypothetical $100/$1 split I discussed with the trading rep?