I would strongly suggest having a look at Sheltons books on fasting, particularly his hygienic system, volume 3 ( its free online, just type those search terms in)
Although the book is decades of years old, it is by far the best source on fasting I have found so far. This man has had a huge experience with it and has fasted thousands of patients himself.
The longer the fast, the more important it is to break the fast correctly ie on very light, liquid foods-fruit juices and such (as not to strain the stomach).
As the previous poster mentioned, one should only take water during the fast, no supplements of any kind. If you can, try and get that vitamin d through sun exposure instead.
Hope any of this will be of some help