It could also be a lacking niacin or vitamin B...i've had ducklings and chicks act the same way and after a vitamin/mineral powder mixed heavy into their water they straighten right out. i also keep sublingual B complex liquid on hand and add it to their water if i see any issues or sometimes directly into their mouth...since it is sublingual they do not actually have to swallow it. i know the OP said they gave vitamins but are you sure you gave enough? This is what we use..but we got a bigger pack in a foil bag http://www.tractorsupply.com/en/store/durvet-vitamins--electrolytes-100gm-jar They also have on specifcally for poultry but tractor supply doesnt sell it....its call High Performance Poultry Pakthat is a hard call, i have two silkies right now that are 'rolling' (their neck/head is curling under-like they are trying to groom their bellies and they then fall foward-it only gets worse if they are stressed, if they relax they will stand stooped but be able to eat/drink)-- and silkies are usually resistant (dont know if it is significant but every one of the 4 silkies that have had symptoms were black), i had one roller last year that got better and was fine-so a mild temporary neurologic symptom. i dont have him anymore but i know they can have relapses. if he does have it and get better the probability of relapse is high, the question is is it mareks? there are other things that can cause that problem, and weaker chicks are shyer and cause some of the issues by hiding. if you could isolate it and offer it food/water without the others it may get better and catch up.