I have two types of multi-vitamin that I use occasionally. One is a powder which I sprinkle on food maybe once a month., the other is a liquid which I put in the water with about the same frequency (not at the same time though).
Whether it's necessary, I don't know, but the vitamins don't stop them drinking the water. Some of the snakes are fussy when powder is sprinkled on their food though, but they're the same with calcium powder.