Sounds like your doing a good job. I would go ahead and vaccinate the birds that did not receive the Marek's Vaccine in this case, it could possibly prevent or minimize risk of Marek's Disease and complications while they are in quarantine for 4 weeks. It certainly can not hurt them and may...
I agree with Muddy 75 with some additional comments. Also would keep in separate housing/run for 30 day quarantine. You can go to article here on BYC regarding flock management for adding new chickens to flock. https://www.backyardchickens.com/articles/adding-to-your-flock.47756/
Note quotes...