If you're doing a water fast, I would not take vitamin C. The idea is to get to ketosis, which takes about 3 days, give or take. Anything besides water will irritate your stomach, causing hunger pangs. The best thing to do would be to let your digestive system shut down so the real work can get done. I have, however, put drops of iodine or some colloidal silver in my water during water fasts. That doesn't seem to bother anything.
If you're doing any kind of juice fasting, best to get the vitamins from the juice.