Adding vitamins to water is honestly not effective.More often than not it ends up making a hamster not want to drink.Powdered vitamins are much better and can be mixed in baby food and they don't tend to deter hamsters that way
However,mushed high quality pellets(Science Selective ideally)would have the necessary vitamins/minerals,so supplements of vitamins and minerals could actually get to an overdosed level and then cause serious health problems.