Hi S.,
I just began giving my 2 year old vitamins that were prescribed by her pediatrician. She really liked them, but we noticed within a month that her sleeping schedule all of a sudden changed dramatically. She would NEVER go to bed until the middle of the night, sometimes dawn! It was killing us. We didn't even realize it might be the vitamins until a friend told me that happens sometimes in children. We took her off, and now she sleeps like a dream again. This will of course vary from kid to kid, so I'm certainly not trying to scare you off of giving vitamins - that was just my experience.
Talk to her doctor, if you trust them a lot, and see what is recommended. As long as a healthy diet is part of her life, vitamins might not be necessary. But advice from other moms might teach us both differently! I'll certainly be reading up on other responses to see if I learn anything new too. Good luck! (I'm sorry, I don't know the name of the vitamin that we were giving my daughter.)
K.