Vinegar (acidic, pH=2) works good in the rinse cycle to make your towels softer.
Here is how and why they work. Detergent is an alkaline (pH=12, the opposite of acidic on the pH scale). When you wash your towels (or anything for that matter) there are small amounts of detergent left behind. When your add Vinegar it balances the pH and helps removes the excess detergent from the wash.
My product, Micro-Restore Detergent (ph=12), is specifically designed to break down and emulsify heavy oils, waxes, and heavy elements that are in car care products, and that come off of our cars (break dust, oils, dirt). Elements that normal detergents don't do a good job removing. In its concentrated form it is very alkaline. When it is diluted in the wash water (water is pH=7) it becomes less alkaline (pH=9 or 10). Depending on how much you need to use (which depends on how dirty your towels are), it may be beneficial to add Vinegar to the rinse cycle to remove any extra detergent that has been left behind.
Vinegar itself will not do a good job of cleaning, in fact I think it will make the cleaning process less effective. Think of it as cleaning away the exess cleaner. Kind of funny that you need to clean the cleaner. Don't use to much though because I think it can have negative effects just like bleach (pH=13), which breaks down the microfibers over time.
I hope this helps explain why vinegar is sometimes a good idea in the rinse cycle. It'll make your towels feel softer, and make them more absorbent.
Ian
Autofiber.com