Vinegar is acetic acid: CH3-COOH. In water it will only dissociate to form acetate (CH3-COO-) and hydronium ions (H3O+ <--- the acid). It will lower the pH of your water simply by the fact that it is an acid, however, i do not know the affects the acid or the acetate ion will have on your fish. I don't understand how this could be used as a carbon source because it does not form CO2 in water. If plants used acetate as a carbon source then it would be plausible to use vinegar as a supplement, however, to my knowledge, that is not the case.
I would advise against it and perhaps use pH lowering regents designed for aquatic use.
I live in Baltimore and my water comes out with a pH of 8.6 and my fish dont mind. My pressurized CO2 drops my pH to 6.7 and my fish dont care.
Vinegar can also break down the kH (I believe it breaks down calcium carbonate, and this can be a source of CO2, but don't quote me on it b/c chemistry was never my strong suit! LOL)... but it wouldn't be enough to use as a consistent source for the plants.
+1 that you probably don't need to change your water at all
However, if you decide you're set on lowering your pH, the best/most consistent way would be to get an RO unit and use that water for your tank.
I learned over the years that if you have high PH high KH water, learn to live and work with it. You can do more damage to fish trying to adjust it, than just plain leaving it alone. Certain fish can't live under these high conditions (and you learn quickly which ones), so you simply don't try and keep them.
From what I've read and learned, plants are more tolerant.
The only time I use vinegar is to clean the glass.