Over the long term, sure - you're being a ridiculous market-timer. But in the short-term, I get the same feelings you do. The market has been pretty volatile lately, and it's not uncommon to have 100-point spreads from open to close. I felt a bit "burned" on a few occasions when I decided to just dump a lump sum into VTSAX, and by the end of the day the market had surged. So at a minimum, wait until 3:45pm before you decide, and look at VTI as a marker for how VTSAX will fare that day. If it's surged, then wait a day or two. If it's dropped (however much)...you'll feel better throwing that money in at a "dip", even if it's just a weekly dip.
I know in the grand scheme of things, that 100 points doesn't matter much over 10 years time, but it sure helps me avoid the sinking feeling I get in the pit of my stomach when I make an irreversible decision that would turn out differently if I could have just waited a few more hours.
I now have an alarm on my calendar to ping me at 3:45 each day to remind me to look at the charts to see if I want to throw some extra money in. Call me a ridiculous market-timer too, but it helps me sleep better at night.