Vitamin deficiencies are a dietary issue and would not just affect a single chick while the rest are developing normally. It is likely that there is something physiologically wrong with the chick that extra vitamins are not going to fix. The fact that it is only this one having a problem indicates a problem only with this one. Some chicks simply do not develop properly. I highly doubt that it will ever catch up or change. It certainly is not just a "slow feathering" gene at work either. I am sorry but it does not look good for the long term.
