This is not really a Marek's issue, but one of mixing chicks with grown chickens. Not a good idea.
As for Marek's, it is likely that your unvaccinated chicks will acquire the "lab" virus from the vaccinated chicks. By lab virus, I mean the weakened strain that is in the vaccine and that the vaccinated chicks acquire when vaccinated, as opposed to the "field" virus, which is the full strength virus in the environment. The vaccine is a live, although weakened, virus. It does not prevent the vaccinated chicks from acquiring Marek's, it just reduces the likelihood that they will be symptomatic, such as grow tumors, etc. The unvaccinated chickens will probably become infected with the lab virus from the vaccinated chicks, but they don't acquire same protection as the vaccinated chicks. In other words, catching the lab virus from a vaccinated chick is not the same as being vaccinated. Therefore, the unvaccinated, but infected (with the lab virus) chickens can still acquire the field virus and be symptomatic. In fact, they can acquire the field virus from the vaccinated chicks if the vaccinated chicks happen to acquire it, even though the vaccinated chicks are asymptomatic.
I am sure I am scratching the surface and may misunderstand some things about Marek's, but I think what I have written is basically correct. I welcome correction, however, should somebody with more knowledge care to chime in.
UGCM
