Thanks K.T. that makes sense.
The best reason to vaccinate is to prevent spreading of a disease through a large flock. If you have only one small group of chickens you can take more of a risk. I had a few of mine from a hatchery vaccinated against Marek's this year, but none of the rest.
I've done some "bad" things like mixing birds of different ages, adding turkeys and geese...and I've been lucky.
but years ago I worked for a chicken ranch in California where we raised 100,000 commercial laying leghorns at a time. My job was actually to vaccinate and debeak (awful) the birds.