Q: Can unvaccinated chickens get Marek's Disease from vaccinated chickens?
A: The Marek's Disease vaccination is attenuated, which means, in most cases, it's not even developed using the Marek's virus. Instead, it's developed using a different illness common to turkeys, but which causes the chickens in question to develop immunity to Marek's. This is the same way the small pox vaccine was developed for humans: the vaccine is actually based on cowpox. However, cowpox provided immunity from deadly smallpox, even though you can't get smallpox from the vaccine.
We know of no reports or studies showing that nonvaccinated birds can be infected with Marek's Disease by exposure to birds vaccinated for Marek's. Similarly, infants that haven't been vaccinated for Measles, Mumps or Rubella, yet, don't get those illnesses by being exposed to their mother who has had the vaccinations (or even when drinking her milk).
That said, do be sure you understand what vaccinations your chickens have had. Most hatcheries only offer the Marek's vaccine, but some offer others. Not all vaccines are attenuated like Marek's is. Plus, if you have your birds vaccinated for say, coccidiosis, that won't of course protect against Marek's or anything other than what it's designed to protect against.