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.
The problem with the Merek's vaccine is that it's a "leaky" vaccine. It protects any birds that have had the vaccine from developing any symptoms but it doesn't stop them from getting it and spreading it to others. That's why I'll never vaccinate for it.
If you or Dennis are able to do it, I can bring the vaccine with me. However, if it's something worse, then I will need wait until all the current girls pass away before I can get any new chickens. I got this black chicken plush for my son, and he holds it all the time when he is awake except his remoate learning time. The poor kid really hates remote learning. I hope they let him back to school soon.
She had minor coccidiosis too, which is not the cause of her death, but Doctor Dunn suggests that I don't need to treat the other chickens. She also said many chickens have high resistance to Marek, but Silkie is not. I explained to her that Candy is not a Silkie, but Ameraucana. Anyway, so I will have to vaccinate my chicks in the future for safe.