If you sell VTSAX before the market opens, then you just get whatever changes happen during market hours. So you made your post at 11 pm - in that case, you'd wait for the next day, for the market to close. I think it then takes one more day for the money from a sale to be available in your account.
ETFs sell immediately, but it takes 2 days to fully resolve the sale. If you want to withdraw money, then you're still waiting 2 days. But if you want to sell one ETF and buy another, you can do that instantly. Vanguard credits your account for the sale, and you can immediately use that credit to buy something else. You can easily do both in under 1 minute - at the current market prices, without waiting for the market to close.
I think some Vanguard funds restrict how often you can buy and sell? They do that because the fund itself deals with your sale or purchase, and they prefer stability. But ETFs trade on the open market, so there's no restriction on frequency. There's huge institutional investors that wait for ETFs to be worth pennies more than the underlying assets, and when that happens they are authorized to create blocks of new ETF shares.
