I've read a little about Marek's changing nature and vaccines driving it before. Definitely makes you pause for thought. I've been vaccinating against it for years and not sure if that's good or bad.
I have vaccinated and unvaccinated birds together all the time and no mareks anywhere. I started vaccinating because I did have a mareks outbreak something like 10 or so years ago. It took a lot of work to rid my property of it and I had to go chickenless for a long while to do it. I don't actually need to vaccinate since I haven't seen mareks in a bird since that episode so long ago.
At any rate, just want to be sure everyone knows that the vaccine doesn't cause Mareks. It does help keep birds who catch it after vaccination alive. So if you vaccinate, and your birds catch mareks afterwards, they can survive and then make unvaccinated birds sick. But only if they catch mareks after vaccination, not from the vaccinations themselves. Having unvaccinated birds with your vaccinated birds helps you to know if mareks makes it into your flock so you can respond accordingly. Either culling or closing your flock.