How do you know it is vitamin deficiency? Have you had a necropsy done?
I'm afraid it is far more likely that you have Marek's disease in your flock as that is one of the commonest killers of chickens.... far more common than vitamin deficiency, unless you have been doing something seriously wrong with your feeding regime. I appreciate many people do not want to accept that their flock has Marek's Disease but it is so widespread you are probably lucky if you don't have it. Because there are some strains that are more virulent than others and some birds are more resistant than others, there are people that just lose the odd bird to it and put it down to vitamin deficiency or whatever and others lose half their flock. Also birds sick with Marek's will often respond to a vitamin supplement because it supports their immune, digestive and neurological system and that helps them to fight the disease. People then assume it was a vitamin deficiency. It does not always work but can give birds a helping hand. It will not cure the disease though and they will likely have another outbreak of it at some point in the future, just like people have with cold sores (Marek's is the same type of virus (herpes), but in between will appear perfectly normal.
I hope I am wrong but I think it is important to know all the facts and be aware that you may have a very contagious disease in your flock.
Regards
Barbara