Personally I've never understood this vaccinating after or the day of weaning.  WE vaccinate months before then booster 3-4 weeks before weaning and they aren't touched again.  The dust from the dry lot will be your biggest enemy right now.  If simply running them down a chute and giving them a shot is enough stress to make them sick there are bigger issues.

Weather will be the next biggest factor, you don't want to wean them when there are going to be major temp swings from day to night.