The Marek's vaccination could help spread the disease. A bird with a vaccination can still get Marek's it just does not kill them, they will still be able to pass it on to other birds and kill them though. Either all of your birds should be vaccinated or none of them should be. When Marek's breaks out in your flock you will separate the infected birds and let them die, or possibly recover in some cases, if the vaccinated bird is infected then it will not die but it will continue to pass the disease to unvaccinated birds. You will either have to kill this bird if you want to stop the spread or sell it to someone who has a fully vaccinated flock. If you are planning to sell the birds after the show then do not vaccinate them, if you have an existing flock that is not vaccinated then don't vaccinate them, if you just will have these birds and you don't plan on selling them then go for it. Just remember, if you want to get more birds to add to your flock then get them vaccinated to. KiKisGirls is right, they will need to be vaccinated right after they hatch.