I would keep him comfortable and fed. Give the vitamins, and you can use chick vitamins (make sure they have riboflavin) or the human B complex to see if his legs get better. If you should lose him at some point, it would be good to get a necropsy (autopsy) by your state poultry lab. The body has to be refrigerated, but not frozen, and taken or shipped overnight. Usually they can tell the cause of death by examining organs fo signs of infection or tiny tumors that Mareks causes. Here is a link where to contact your state vet:
http://www.metzerfarms.com/PoultryLabs.cfm
http://www.metzerfarms.com/PoultryLabs.cfm