Chickens can still get Mareks after having the vaccine, because it may take 2-3 weeks for the vaccine to take effect. It also affects chickens his age. Vitamin B2 (riboflavin) deficiency can cause curled toe paralysis, so it is a good idea to treat with riboflavin, or vitamin B complex 1/2 tablet daily, even though Mareks could still be the problem. Mareks can best be diagnosed after death with a necropsy, so I would contact the state vet for one if you lose him. Be sure to refrigerate the body before taking or sending it for a necropsy. Here is a link about state vet contact info:
http://www.metzerfarms.com/PoultryLabs.cfm
http://www.metzerfarms.com/PoultryLabs.cfm