The only way to keep your entire flock vaccinated for Marek's is to purchase vaccinated chicks from a hatchery and keep them quarantined for at least 10 days or to hatch your own in an incubator and keep them quarantined. Most feedstores do not sell vaccinated chicks - vaccinated chicks cost more money and can't be safely quarantined at a feedstore. So.... those of us who purchase chicks from feedstores or let our hens hatch eggs will be raising unvaccinated chickens.
Typically, Marek's strikes chickens between 12-24 weeks of age. For those of you who are worrying about your unvaccinated older chickens; you don't need to be concerned about your older chickens getting Marek's disease once they are at least 7 months old (it could happen, but it's rare). You can always vaccinate your new additions to the flock to keep them safe as long as you hatch them yourself or buy vaccinated chicks from a hatchery.
My flock of 21 are unvaccinated. I have 13 that are safely past the age of getting Marek's. I did lose one pullet about 3 months ago at 16 weeks of age, but never determined the cause -- all the rest of the chickens have been very healthy. I hate the idea of losing any of my chickens so I'm going to be extra vigilant in making sure my 8 younger pullets (now 14 weeks old) are not stressed during the danger zone. That means making sure they get extra nutrition like yogurt, greens, etc, and not integrating them into the older flock until they have started laying. Integration is stressful on newcomers and I think Marek's tends to attack their systems when the chickens are getting ready to start laying.