I'm not completely sure I understand the question. But do you know that funds don't fluctuate throughout the day? A mutual fund has one Net Asset Value (NAV) calculated daily after closing. Any purchase orders placed that day will get the NAV calculated after closing that day.
What you want is the ETF. An ETF is a mutual fund that trades on an exchange, so you can get price changes throughout the day.
does anyone else cherrypick down market days to buy VTSAX, and avoid buying on positive days?no way to tell if it makes any amount of difference long term, but maybe psychologically makes you feel like you're gaming the system