The typical time for Marek's to strike is anywhere from 3 weeks to 6 months, though 8-12 weeks is most common. If you got them in spring then they should be nearing six months which means they are unlikely to show symptoms even if they did contract it.
The main issue here is that the birds are simply too old. The vaccine will not take in their bodies. Their systems aren't likely to even notice it. Antibodies are something built as young chicks; their bodies are no longer in that critical phase of building.
Beyond that, the Marek's vaccine is very hit and miss. The vaccine itself is simply the turkey version of the Marek's virus. While it can't make chickens sick, it's close enough that their body recognized it and attempts to fight it by making antibodies. Note that 5% of birds, even when vaccinated, will not be protected, as their natural immune system simply isn't strong enough to fight off the disease. Also note that the vaccine does NOT prevent birds from contracting Marek's, only from showing symptoms. They can still carry and transmit it.
