While I did not have any difficulty with a recent Roth conversion at Vanguard, according to the pop up that displays on their web site and The Independent Advisor for Vanguard Investors, there have been numerous people who cannot complete conversions transactions and RMDS in the last week or so. This is a real problem if you are trying to ge tit done before the end of the year.
I will say, Schwab made tax withholding on the conversion/distribution painless and on line. Vanguard required a complicated form with many jump through hoops and final signature on Docusign. I said forget it and just bumped up my estimated tax payment.
I am finding fewer and fewer reasons to stay at Vanguard, other than one Admiral fund I have.