I know that there are lots of people who use vinegar because their mothers used it, their grandmothers used it, and it "has always worked". Vinegar does have its uses, including in the laundry to combat odor. The problem is that it doesn't set dyes in cotton fabrics. It's the washing that's getting rid of the excess dye, not the vinegar.
This article explains the principles involved:
http://www.pburch.net/dyeing/FAQ/settingdye.shtml
Yes, you can still find multiple sites on the internet that claim that vinegar works. It's an old wive's tale!
This reminds me of a story that Bob of Superior Threads tells, which can be found here:
http://www.superiorthreads.com/educa...dition-or-myth
Just because it's always been done that way doesn't mean it's effective.