The Mareks vaccine is a live virus that creates an immune response in the chick so that when it encounters one of the disease-causing strains of the Mareks virus the chicken doesn't get sick. It's about 90% effective so some vaccinated chicks will occasionally develop Mareks disease because their bodies don't effectively fight off the "real" virus even after exposure to the harmless one.
Your vaccinated chickens can still contract the Mareks virus and can shed it in their dander, just as unvaccinated chickens can, but other than that they pose no danger to unvaccinated individuals and they'd have to be exposed to actual active virus first for that to happen, it will not come from the vaccine itself.
The reason for quarrantining vaccinated chicks for 2+ weeks is to protect them from exposure to the Mareks virus until after the vaccine strain has gotten a running start in their system. In other words, you don't want them to be exposed to Mareks until after they've developed some immunity to its effects. An unvaccinated chick doesn't need to be protected from a vaccinated chick for danger of catching Mareks from the vaccine. I believe Mareks lives in the ground and it's exposure to your other outdoor birds that is more of the issue.
I mix my chicks all the time. However you cannot account for any other virus or bacteria that the
TSC chicks, or any other purchased birds, may have but they cannot give Mareks to your unvaccinated chicks. You can also if you want buy the Marek's vaccine and immunize your own just hatched birds. I have also done this and it's relatively easy also.