Boundary confusion here.
When StockCharts shows a heading of Dec 7, 2018 - Sept 24, 2019, it means the performance on those days,
inclusive. So if there were no dividends, one would just take the closing price on Sept. 24 and divide it by the closing price on
Dec 6 (and multiply by $10K) to get the final value of a $10K investment.
StockChart graph for VTSMX.
Since the fund had divs, you can use the adjusted prices from Yahoo to verify that this is in fact what StockCharts is showing (correctly). FWIW, I cross checked Yahoo's daily prices with Vanguard's data, incorporated the divs from Vanguard, and came up with $11,370. Clearly some rounding errors by someone, but close enough to validate the StockCharts price.
M*'s handling of the boundary dates is a bit problematic. The chart
linked to here will illustrate. It graphs VTSMX from Dec 7, 2018 to Dec 10, 2018. (Weekend days were Dec 8 and Dec 9.)
Notice that there are three prices in the graph, i.e. two changes. Since this spans a weekend, that must mean that it is including the performances of Dec 7 (Friday), and Dec 10 (Mon). But it shows the starting value on Dec 7 as $10K, and a drop (to $9770.83) on Dec 8th, a Saturday. After one day with no price change (for the weekend) it shows a price change on the 10th. But a weekend must have two days with no price changes.
Regardless, it would appear that M*, like StockCharts, plots performance
inclusive of the start date. It is just struggling with how to represent the change on the first date. (In fairness, StockCharts doesn't do any better; here's
its chart going from Sept 23 to Sept 24, and it shows only one price change.) Or perhaps not ...
Rather than waste time reverse engineering how these tools handle their start points over different time spans, I'll just suggest you look at
this M* graph of VTSMX from
Dec 8 to Sept 24. Dec 8 was a Saturday, so in theory this should make no difference (but it does). This graph shows a final value of $11,371, within a dollar of my calculation and a couple of bucks or so of StockCharts.