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: 
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.
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