Mareks tends to affect young birds in a flock typically between 2-8 months old. Some birds or breeds seem to be more susceptible to it. Some birds seem to be immune to it. The reason I mentioned Mareks is that you have said that you have lost young birds to suspected vitamin deficiency. You said your bird has the splits. The B complex should be injected at the base of the neck if you are giving it under the skin. You can also give B complex orally which is safer and easier. Sometimes it is good to post a short video of the chicken walking to YouTube with a link posted here, to see what you are seeing. If you should lose another bird, I would have the state vet do a necropsy to look for what killed your bird. No one wants to believe that they have Mareks, but it would be a good way to rule it out, and get you some answers.