I know that a Water Fast suppose to be pure and really add nothing to it. So therefore your body can work at it's optimal level at cleansing a removing heavy mucus from your stomach lining so that when this is done and you return to taking vitamins it will be more effective, with you seeing and feeling the difference and the absorbtion is greater.
Although I did not do this during my current water fast. I did buy some flavored bottled vitamin spring water at the grocery store.
Just to let you know , Have fun and love what your're doing.