Aim small, miss small bud.
Pick a hair on the area of the vitals, and aim for that hair. True, you probably can't actually see a single hair, but you can pick a small point to aim at, which will help you get your arrow going where you want it to.
Think of it like this - you're aiming at a soccer ball - would it be better to aim at the whole ball, or would it be better to aim at one of the dark spots near the center?
I'll bet dollars to the hole in a donut that if you try it on a target before you go out, you'll be able to do it on a deer too.
Good Luck and God Bless!