Did you happen to check late at night? Vanguard has some maintenance going on at night, and rarely I'll see information isn't shown while my account is being updated. That may have changed - that was years ago. In any event, I think cost basis info only gets updated once a day (even for ETFs).
A nice feature of ETFs: you can sell them in real time. This week has lots of -3% days. With ETFs, you know what happens if you buy or sell. With a mutual fund, the market will keep moving for hours after your order is placed, and you get whatever the result is of those price moves.