I never used any medicated feed on my birds, nor have a vaccinated any of them. That is all up to you and what you feel is necessary. My thoughts are that my chicks needed to grow up as natural as possible.
If they are on a good feed, then no added vitamins are really needed. However I occasionally add probiotics to the water a few times a month for healthy crop bacteria, along with ACV in the water a few times a month, also for crop health. Tomatoes everyday to keep the crop ph low. (the crop seems to be the bull's eye for issues with poultry. LOL At least in my flock)
You might want to get some Duramycin antibiotics to have on hand just in case somebody does get sick and needs treatment. Duramycin is a broad spectrum antibiotic and will take care of most issues with chickens.