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: 
If it contained a high amount of rye flour, an acid would be needed for the bread to leaven. This is because bread with lots of rye rises due to polysaccharates called "pentosans" (if i remember correctly) being sticky and holding in the carbon dioxide bubbles. With heat, an enzyme in rye called "amylase" will start eating up the pentosans, unless the amylase is deactivated with acidity (wheat flour uses a protein called "gluten" to trap bubbles, and its amylase is deactivated with heat anyhow). The acidity is traditionally lactic acid, produced by lactobacilli bacteria in sourdough, but could be vinegar.

If it's mostly wheat flour (i.e. if you have to knead it), the vinegar would just be for flavour.
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.