[Peter Finke]

To lower pH by vinegar is really nonsense and contraproductive. You can use pure acids (as phosphoric acid or sulphuric acid in addition to humic substances from peat or leafs) but not vinegar! Vinegar is food containing many different aromatic ingredients and produced from many fruits or herbs or other things (see http://en.wikipedia.org/wiki/Vinegar ). You do not want to aromatize your water but to lower the pH.
Indeed, you could use some sorts of acids, and there are industrial products for this (e.g. based on oak). But vinegar, really, that is to mistake the aquarium for the food table!