VA, or Volatile Acidity refers to a set of specific types of acids in wine. When imbalanced (when any of the volatile acids are dramatically recognizeable) it becomes a wine flaw, and the wine noticibly tastes or smells like vinegar or nail polish remover.
All food items have acidity to make them palatable. One look at a pH table shows how what we eat varies in acid content. There are many different types of acids, including the Malic and Lactic acids which are ever-present and vital to the enjoyment of wine. Malolactic Fermentation is the common process of using a certain bacteria which eats one and turns it into the other.
But the presence of the “volatile acids” Acetic Acid and Ethyl Acetate (and others) give a flavor and aroma of vinegar. In fact, this is the method for making vinegar! Usually, it occurs as an infection during the winemaking process. This specific bacteria only grows in oxygen and when sulfites aren’t present, especially when care is not taken to remove moldy grapes from harvested crops, or to keep a ferociously clean winery.
If you taste or smell vinegar, it’s important to know if air & bacteria got into the wine through a faulty cork or storage. If it got in during winemaking, there’s little hope that other bottles will be any better, and there is reason to suspect all other wines from the same winery!
If a bottle of wine smells or tastes strongly of vinegar, you should return it immediately, and only if you purchased it within a few days prior. Select a different wine entirely as its replacement, unless you’ve tasted or know other bottles have been just fine.
Would you like to know more? There’s a complete and more scientific discussion of these acids at the website for UC Davis