Why do some pie crust recipes include vinegar?
From On Food and Cooking:


  Acidity in the dough - as from a sourdough culture - weakens the gluten network by increasing the number of positively charged amino acids along the protein chains, and increasing the repulsive force between chains.


And weaker gluten structure is definitely a good thing for pastry doughs!

From the same source:


  [Eggs] supplement gluten structure with tender protein coagulum, tenderize products; slow staling


So eggs help provide a more tender structure, again good for pastry doughs.