How do i shrink 100% cotton shirts?

Wouldn't the vinegar make the shirt stink

The best way to shrink 100% cotton shirts is HEAT. Luckily for you, cotton is the easiest fabric to shrink. Just wash and dry the shirts using the hottest option available, both for washer and dryer. This should work nicely.

i have a black t shirt with glitter that my daughter just bought and sent tome. it's about 2 sizes too big, will the hot water soak with vinegar affect the color or the glitter

You can use following tips to shrink cotton fabrics:
Wash your shirt in hot boiled water.
Or soak cotton T-shirt in boiling water.
Remove your shirt and dry it on the dryer.