Not to startle you, or break your sense of security, but your chickens aren't safe from Mereks, even given your circumstances. Its often carried by wild birds and you can pick it up on your clothes and shoes just by walking where others with infected birds have been.
That being said, I don't vaccinate either.
X 2. Marek's is worldwide. If you have chickens they have likely been exposed at some point. From wild birds, rodents, the wind, human contamination and so on. Just because there are no other chickens near you, or none of your current chickens have ever had symptoms, that doesn't mean Marek's can't be or hasn't been present. Even the vaccine doesn't prevent a bird from being infected with Marek's, it only gives the immune system a jump start to prevent tumors from forming. More information can be found here:
I am also another that does not vaccinate.