Another prewasher here. I always prewash all of my fabric. The worst of the shrinkage happens from the heat of the dryer but the moist heat is important. I always "over dry" to make sure I get as much of the shrinkage out as possible.
For the vinegar, I use a quarter cup in my front loader. I used half a cup in a full load top loader. I go through a fair bit of vinegar with my laundry in general too. It helps keep less than stable colours like black and red looking new when you use it in every (or most) washes. It also will take musty smells out like if a damp towel got wadded up and forgotten for a while. Also helps keep your whites really white without giving them a yellow bleach tinge.
Tara