Vinegar is a sign of acedic infection. Insufficient sanitation is the cause. Fruit flies are known carriers.
+1 Some info i put together for our club off flavor tasting.
Definition: Acetic acid is a compound that causes off flavors and aromas in beer, often described as a sour or acidic taste or a vinegar character. In fact, acetic acid is the active ingredient in vinegar.
Characteristics: This is one of the five basic tastes, and is often perceived the most on the sides of the tongue, towards the rear of the mouth. The two most common acids responsible for this flavor are lactic and acetic, which both have related esters that may be perceived in the aroma. Appropriate for Witber, Lambics, Flanders Ale and Berliner wesse. Optional sourness ok in American wheat, Rye, Dry stout and saison.
Causes: Lactic acid is produced by Gram positive bacteria such as Lactobacillus and Pediococcus, which are present in dust and saliva. Acetic acid may be produced by several contaminants, including Acetobacter, Zymomonas, and yeast in the Kloeckera and Brettanomyces families. High levels of sour and acidic flavors generally indicate a sanitation problem, but they are an important part of the profile of the lambic, oud bruin and Berliner weiss styles, and to a lesser extent, Belgian white beers. The most common causes are poor sanitization; bad yeast strain; too much corn sugar; excessive amounts of citric or ascorbic acid; high fermentation temperatures; excessive acid rest; mashing too long; use of wooden spoon in cooled wort or fermentation; storage at warm temperatures; scratched plastic bucket/furminator.
Management: Good Sanitization Use Glass or stainless steel equipment and spoons. Cool fermentation temps and storage Keeping mashing times below 2 hours.