You cannot categorily state that someone who makes vitamin C is more likely to survive than someone who needs to eat it.

Genes work in weird and wonderful ways. It would not be inconceivable that the mutation in the gene that led it to no longer produce vitamin C led to it producing some other protein that, in fact, produced some greater evolutionary advantage that we do not yet fully understand. Well, it can't be that bad if we've survived this far, right?