As I said before, I use vinegar in the rinse cycle but you could put it in at the beginning along with the detergent. I don't think it makes much difference (as long as there are no tough stains) but I like the added rinse cycle so that is why I wait for the entire washing cycle to be completed, then add vinegar and rinse again.
Vinegar makes the towels softer and helps retain their "fluffiness" but it does not necessarily aid their cleaning. In fact, you need to be sure the towels are as spot free as possible or the vinegar could actually "set" the stains in the towels. That is another reason for washing first, then adding the vinegar to an extra rinse. Basically, vinegar acts as a fabric softener without the static. Also, if you toss your wet towels in a box or hamper and they sit there a week or two until you get enough to justify a wash, the chances are pretty good they will develop a mildew or chemical smell. Vinegar helps eliminate that smell better than just detergent.
I've heard that using baking soda is another thing you can do to help rid the towels of soap residue and helps make your towels softer but I have not tried that. Anyway, I think the bottom line is that using vinegar may not be a necessary step in caring for your towels but I think it helps in the long term.