It's my understanding that ACV will only work for you if your problem is the volume of acid you are producing. If your acid levels are normal and your problem is primarily a weak oesophageal sphincter, then it wont work (nor will PPI's!).
The theory that reflux is caused by not enough acid in the stomach is half true... what happens is we eat foods these days that are hard work for our guts to digest, and we aren't producing enough acid to do this. In response our body produces more, to digest that food until its gone. The problem is, it then continues to produce at that level, even when our stomachs are empty, and that's when we have too much and get reflux. The idea with the vinegar is that it fools the stomach into thinking its produced enough acid to digest the food, and so acid production backs off a little. The acetic acid in the ACV is less potent than our HCl acid in our stomachs, so we process it much quicker and easier and it doesn't have chance to reflux.
That said, it never worked for me, as my problem was bile reflux, so all the vinegar and PPIs in the world couldn't fix it. Shame it took 'em 10 years to figure out, so now I have Barretts and have just had a Nissens!!