This doesn't work with Vi's Q (if you mean releasing Q then trying to Flash during it, like you can with Shen's E). What you can do is walk in while charging up your Q, then Flash, then release Q (very quickly so they have little time to react). Even at extremely high ELOs few can dodge it if executed right. Of course, you can only do it once before you hit 6, so you gotta make it count.
Some more info:
If you want to learn vi's flash q, you need to remember the distance of vi's fully charged q and flash by heart.
What basically happens with a vi flash q is this:
You walk into lane charging q
You let go of a fully charged q to start dashing
You flash on the enemy mid dash to hit a fully charged Q
It takes a lot of practise and experience with vi to pull off. Its pretty much the ceiling to vi's skill cap along with when you want to flash mid ult to avoid the enemy dragging you into their whole team with a blink ability.
If you flash before you release Q, you give the enemy time to react. You dont want that.
