|Vinegar use isn't an exact science, and whether it will help or not will depend on your water. That said, I do toss in a glug because I think it helps my diapers be a little softer and rinse a little cleaner. My chemist DH says that in theory it would be most effective in the hot wash with the detergent, to help keep the detergent-mineral complexes (that lead to build-up) from forming and sticking in your diapers, so I guess that would probably be the bleach spot. Anecdotally, I think that most people put it in the fabric softener or rinse spot. Personally, I use it in my pre-wash, because I am now washing urine-only night diapers, so do a pre-wash with diapers and things like dishcloths, muddy socks, kid-napkin-cloths, etc. and like having somthing to throw in there with them that isn't detergent, since they'll get that when I throw in all the other whites to join them in a hot wash.
As for amount, I've seen anywhere from about 1/4 cup to 1 cup recommended for regular use. I probably use about 1/4 cup, but have to say I'm not measuring it, just putting a glug in.