Your soils pH is determined by the free hydrogen ions, or lack of, in the soil. The more there are the lower the pH and the fewer there are the higher the pH. It takes a lot of lime, Calcium Carbonate, to make a small change so 1 cup of vinegar mixed into 1 gallon of water will not doo much of anyuthing if spread over more than about 1/2 square foot of soil.
Contact your state universities Cooperative Extension Service office about having a good, reliable soil test done.

Here is a link that might be useful: About soil pH

Kimmsr's link above it to a rather good site. Also you may want to consult Soil pH - Wikipedia, the free encyclopedia.

Jbander: by all means look into the soil science before proceeding. Different soils have different capacities to buffer (resist a change in) pH. your recipe in a very sandy soil will likely have a much greater affect than in my calcareous clay loam, complete opposites as far as pH buffering capacity goes.

And then there's the water part of the equation and buffering. I'll leave it at this for now..... I inject vinegar into my irrigation water to deal with the bicarbonates in it to drop the water from pH 7.4 to 5.6. First, 750 ml of vinegar is added to 5 gal water for the stock solution which is injected into the water at a 17:1 ratio (17 water:1 stock solution). It should be pretty darned obvious to you without doing any math, based on what I'm doing that you are probably creating a pretty darned acidic solution by your method. Of course, my methodology with your water may not yield the same pH change as water too has it's own pH buffering capacity.