Salt is actually way more important in the short term than vitamins. Add a pinch of salt + lo-salt to every alternate glass of water. Magneisum maybe useful too. Vitamins maybe useful when you are doing much longer extended fasts (>5 days).
I say it depends on your body fat lvls, eating habits, regimen, But for me I just took water fast as only water simple, 24 days last year, 25 thisAnd now I m learning about Himalayan salt and vitamin pills, ppl. Even Ramadan, so many choices..