Crested birds can have some neurological problems. A peck to the head could do it.
I would still try the vitamin therapy and offer wet feed to see if there's any improvement.
Can you get a video of the chick's actions?
What are you feeding including treats?
What's the poop like?
Photos of housing/where they are kept.
It would be hard to know if the issue is Marek's disease unless you lose the chick and have formal testing done through your state lab.
Losing...