Your choice.
A snake fed a diet of healthy, well fed feeder mice can grow, live and breed without vitamins and he healthy.
A snake given vitamins and a healthy feeder diet can also be healthy.
The only drawback is make SURE you do not overdose on vitamins as this can be dangerous. If you are putting a bit of vitamin powder on mice I would say do it in moderation! Not every single feeding, not large amounts.
bmm