Vitamin synthesis is one of the most pointless things to add because it is something that is lost in many lineages over evolutionary time. The reason is that as

Vitamin synthesis is one of the most pointless things to add because it is something that is lost in many lineages over evolutionary time. The reason is that as long as there are environmental sources of a molecule the pressure to retain the ability to synthesize that molecule natively is zero or even negative, because the synthesis pathways for some of those molecules are quite nasty. You could do it, but those people would probably have increased cancer rates due to running nasty biochemistry in their own cells and eventually their descendants would lose the ability the synthesize it again because vitamin C is naturally present in their environment unless they are e.g. British sailors stuck on a boat for months with a evolutionary strange diet.