Adding acid to pie dough in the form of vinegar chemically changes its makeup, creating a crust that is more tender and flaky. The amount of vinegar in pie crust recipes is small enough that it does not affect the flavor.
The addition of water to flour creates gluten, strands of protein molecules that become too long and elastic, resulting in a tough pie crust. The acid in 1 tablespoon of vinegar, along with the shortening, interferes with the elasticity of these long protein molecules, breaking them down into smaller strands to make the dough less elastic and flakier. Vinegar also keeps the crust from becoming too brown, especially helpful with a pie requiring a long baking time. If vinegar is unavailable, bakers can incorporate acid into the dough by using 1 tablespoon of lemon juice, wine or buttermilk. Sour cream can replace both vinegar and water.