We start deworming our foals at 4 weeks of age--with ivermectin--and then dose them monthly from there on. By the time January rolls around they are about 7 or 8 months old and at that point we put them on the regular 2 month schedule that all our horses are on. I say two month schedule, but sometimes it ends up being 3 months between doses. We do use ivermectin all the time and haven't had a problem with resistance. About 5 years ago we did have a vet tell us that there is some roundworm resistance to ivermectin and that we should use strongid (whichever version of strongid it was, I forget now what the letter designation was) instead for our foals. We did use strongid once that year and got nothing. We dewormed again 2 weeks later with ivermection & got ascarids. So, if some farms have roundworm resistance, we don't, and we have gone back to the ivermectin for the foals.
As for vaccinations, we don't vaccinate the foals. At one time we did give them their two shots of EEE/WEE/tetanus after weaning, but now we generally don't bother until spring of their yearling year. By the time the foals are 4 months old (the earliest I would vaccinate foals) our mosquito season is over so there is no need for the WEE/EEE. Tetanus--perhaps they should have it at that age, but we wait & give it to them as yearlings. If one were to get a puncture or cut prior to that we would have to get some antitoxin to give and would probably give toxoid as well at that point. We generally don't geld colts until they're a year old--if we were to geld a weanling then he would get his tetanus shot at that time.