Water-only fasting is often considered a spiritual journey, in a similar way to other means of achieving altered states of consciousness, such as Grof Holotropic Breathwork or Ayahuasca. Purging or vomiting is considered normal/necessary with Ayahuasca. It is part of letting go of what your body does not need. You can see some of this happening The Sacred Science documentary http://www.thesacredscience.com/.
I have also found that coffee enemas during fasting is a great help. It helps the liver to produce bile and flush toxins more rapidly. Within 30 minutes it will alleviate pain, and it will provide a boost of energy. I use the technique for the first part of a fast when the toxin levels are higher. I endeavour to do the last week without, and only to use distilled water to drink.