Question: Yesterday I made bread based on a recipe I found on the flour bag. It contained both rye and wheat flour but also three tablespoons of vinegar. I've never seen it used in bread before, why is it there?

Answer: 
In making faster no-Knead bread Jim Lahey suggest adding 4 drops or up to 1/4 tsp of red wine vinegar to the basic recipe. Improve gluten development and flavor. See the video with Mark Bittman here:
<http://www.nytimes.com/2008/10/08/dining/08mini.html?ref=dining>
It might also, in addition to something like bicarbonate soda act as a raising agent for some types of bread - the reaction of the vinegar with the bicarb causing gas release causing rising.

This is only a guess however