I was wondering about that too. I know vinegar is used in yarn and fabric dyeing (also Easter eggs!). I looked it up and found this scientific explanation:
"An acid such as vinegar or citric acid is used as an auxiliary chemical to reduce the pH of the dyebath for dyeing with acid dyes, but acid is not in itself a mordant. It is a coincidence that some chemicals that are used as mordants, such as tannic acid, have the word 'acid' in the names; it reflects an unrelated aspect of their chemical structures. Vinegar and citric acid, like sodium carbonate, are not mordants, though people who do not understand dyeing sometimes refer to them as such. Acids work as dye auxiliary chemicals by
reducing the pH of the dyebath to allow the protein fibers to ionize, which aids in allowing them to form hydrogen bonds to the dyes."
http://www.pburch.net/dyeing/FAQ/mordants_and_assists.shtml
I've started using vinegar in my dunking process in the final rinse. It seems to give the leather a softer finish, but it's hard to prove that was due to the vinegar. Maybe it was because I used less soap that time? Every bag, and every dunk, is different.