From what I have seen/heard, Mareks vaccine doesn't do much, it does not keep the bird from getting the disease OR from spreading it once it has it. What it DOES do is mask the symptoms and allow the bird to live much longer with the disease than would naturally happen and so they can actually spread the disease further than if they just dropped dead fairly quickly after catching it. From everything I have read, I wouldn't purchase a vaccinated bird if I had the choice because I would rather they die than live and spread the disease for years. My personal opinion.I am very comfortable with the source breeder. I just worry that he didn't come from a NPIP large hatchery so he never would have received a Mareks vaccine.
Only time will tell if he became my second biggest chicken keeping mistake(#1 was ordering a straight run to start a flock).