That's been my policy also. Some birds have a strong immunity against Marek's symptoms, but nearly all chickens carry it, according to the Merks Veterinary Manual. Vaccinated chickens are just as likely to be infected, the vaccine only greatly reduces (but doesn't totally eliminate) the chance of showing symptoms. As such, the vaccinated birds can carry more virulent strains that would otherwise die out (as it kills off all its hosts). That makes me very wary of "rescuing" factory farm birds, as they are always vaccinated and exposed to thousands of other vaccinated birds - who knows what is brewing in those facilities.
I had my lavender cochin bantam chicks vaccinated at the hatchery, to be sure I could keep a breeding group alive. Their chicks are not vaccinated and if I raise a lot, only very hardy ones will survive to be my more resistant breeders in the future. That is a harsh assessment from a farmer, not the response of a backyard chicken owner with just a few pets.
That said, I am considering vaccinating next year, as most of my customers don't care much about their birds contributing to a long term resistance gene pool.
If this is Marek, your plan seems work well. My other girls are all healthy now. I don't have problem to buy vaccine, but I don't have the capability to give chicks shots. If you don't mind to do it, I can just bring the medicine with me and let you do it when I get more chicks from you.