These birds are very young for Mareks - possible, but a bit unusual. Of course a lot can go wrong during vaccination - improper storage, mixed too long etc etc etc. Some birds will get the virus in spite of vaccination. Is it possible they have cocci? Coccidiosis can also knock a bird down with Mareks-like paralysis and these are young enough it's the first thing I would suspect aside from a poisoning of some kind. Hopefully you will be able to obtain a necropsy on one of the birds and get a firm diagnosis. NPIP does not test for Mareks - in our state, just AI and Pullorum.