How does consumption of vinegar affect food chemistry?
In the case you're talking about (just using vinegar to taste), there should be no effect whatsoever on a healthy stomach. The stomach already contains a significant amount of acid, and a little bit more from the acetic acid in vinegar won't change much, just like eating some citrus fruit with a meal (oranges, lemons, and whatnot) won't change much. Acid-mediated hydrolysis of chewed food is only the first of many steps in digestion - a significant portion is performed by enzymes and commensal microorganisms in the gut. Changing the type or amount of acid by a small amount won't affect anything significantly in the long run.

The only exception I can think of is for people suffering from peptic ulcers, whether from over-use of NSAIDs (Non-Steroidal Anti-Inflammatory Drugs such as aspirin, ibuprofen, naproxen, etc.), infection with H. pylori, or possible genetic/developmental issues (most often seen in young children). In these cases, consuming an overly-acidic meal may aggravate the symptoms of peptic ulceration.