The urge to vomit - I would defer that question to others. I honestly don't know.
I always suppressed the urge with cold water, rest, and deep breathing. However, a friend of mine ALLOWED her body to vomit, and she got a LOT of toxins out (with bile) and felt "a whole lot better afterwards and glad for having vomited"!
So, I guess the decision's yours to make.
(Click on my name and look back to the Spring of 2007 to read about my fast - it lasted 32 days but I get into why I learned that I SHOULD have quit on the 28th day)...the first many days of my journal were lost to a "curezone website crash" - but there's some good info there).