I would say that's probably a bit young for Marek's.. It's more likely to be mechanical damage perhaps to it's neck as a result of being stuck in the feeder. I would isolate it and keep it warm, but if it doesn't improve in 12 hours or so, you are probably best to euthenase. Even if it is Marek's, there's not a lot you can do except be extra careful about bio security between these chicks and your own flock. The fact that they have been vaccinated does not guarantee that they won't get it but makes it less likely to be fatal if they are unfortunate enough to contract it.
Hopefully it's just a temporary paralysis from being trapped in an awkward position.