So Vergil was cured of his super mutant problem if you went that route in the game. why can't you get suringer ammo that would kill or cure supper mutants from Vergil since he was able to cure himself. I know the problem with different strains of F.E.V. but that would be why it would kill any that it doesn't cure.

It was the only cure hes making more he said it might take years to do so

It was also for a specific strain of FEV, the one he used on himself, which was not the one used on the other super mutants out there.