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: 
Another reason you can use vinegar in a bread recipe - to produce a sourdough. Under traditional methods of making a sourdough bread, one keeps a 'sour' (sponge) or a piece of dough from the days previous production, which acts as a starter for the current bread. In many of todays commercial bakeries (grocery stores, etc.), sours are not kept from day to day, and powdered substitutes are used. They are normally just a glorified acetic acid (in a powdered form). So, you can just use vinegar to produce the same results.
I've found that a lower pH (more acidity) weakens the gluten and makes the crumb less chewey or less rubbery.  

Certainly one thing to try before taking the step of adding vinegar to a bread recipe is to replace any bread flour with general purpose flour instead -- it should have the same effect.