Quite a few private breeders do vaccinate chicks against Mareks but commercially bred pullets will also have a range of other vaccinations, as Chuck says, eg mycoplasma. In fairly rare cases, mixing vaccinated and unvaccinated birds can lead to problems, though in practice many people, myself included, have mixed flocks with no ill effects .
I think there are so many things that can go wrong health wise when new chickens are introduced that as much, or more depends on the health of the young birds concerned than on vaccination status. The stress of moving home often can cause pullets to go down with infectious illnesses they otherwise might have resisted, especially Mateks and bronchial infections, so a period of quarantine and carefully-managed introductions to the older flock can do much to avoid problems. If the new birds can be bought from a reliable breeder, brought straight home and left in peace to settle in, they will have a better chance than if, for instance, they are bought at auction, with all the upset and stress and risk of cross-infection this might cause. This is why I wouldn't buy those pullets Bickerton recently told us about, sold from a garden centre, and probably living a stressful life in temporary surroundings before being sold on yet again to a new environment.
Mixing vaccinated and unvaccinated birds is one of those questions where poultry keepers tend to divide into opposite camps, some sadly as a result of hard experience, one way or the other. Personally I'm in the middle - and unless you intend either to always keep vaccinated hybrids, or only to keep unvaccinated purebreds, you ill just have to source your birds very carefully and do all you can to support them in the early weeks with you. You shold, of course, do this anyway, but I don't think the main problem lies with vaccination status in itself, more with the health of the new birds and of your own existing flock.
If you use the Search box you will find some old threads on this topic to browse, if you are interested.