It can be added to slow down the fermentation, it can be added to help insure that the acid forming bacteria predominate quickly, it might be added, I suppose, if one had alkaline water with a higher pH.

That amount of vinegar might lower the pH a notch or two, but isn't going to stop the fermentation process.

vinegar is just a different type of fermentation. You can't get vinegar without fermentation. Vinegar can be made from wine, you just add acetobacter to it.
http://www.versatilevinegar.org/faqs.html