Could you use another type of acid, say hydrochloric? This is more of a monetary issue. Where can one buy larger quantities than 1 gallon of the vinegar?
Besides the Natural Way's imperative to use the mildest available inputs, part of the idea behind using acetic acid is that the unevaporated spray remnants feed the sprayed area. If I had to choose a commonly avaliable mineral acid, I probably would choose sulfuric unless the soil has a high sulfur content. I would not use HCl or other halogen acids. If you choose to use mineral acids, be careful with the dilution. Generally speaking, a 10% sulfuric acid is much more dangerous than is a 10% acetic acid solution. As for where to buy larger quantities of vinegar/acetic acid, where are you located?