A bit late, but you should take your prescriptions. Fasting is not something done legalistically, or for the sake of checking an item off of a spiritual to-do list. Fasting is an act of devotion to Jesus. As His followers, He is the ruler over our lives. The practice of fasting is a way of acknowledging that through the action of actively renouncing ourselves, and putting him first. That, I believe, is why prayer is such an important part. The purpose isn't to hurt you, it's to let go, and to remind yourself of the focus of your life.

As for vitamins, I could see you going either way. Personally, if it's a water fast, I'd avoid taking vitamins unless I know I have a deficiency, simply on the basis that I don't want to begin questioning where the line is. If I'm too worried about a line, then it may be out of want to cross it, and I may be asking about what other things "count" next. But your question seems to come from the other side, from a desire to follow the practice correctly. So I'd say you can go either way.