Whisk together all of the dry ingredients, reserving a few tablespoons of the flour. Work in the shortening, mixing until the mixture is the texture of sand and all the flour particles are coated. Now cut in the butter, but do not completely Incorporate it. You want small pea sized dots of butter, this will add flakiness.
Sprinkle the vinegar and ice water over the dough while tossing with a fork. Just as soon as the dough becomes cohesive (i.e., you can squeeze it into a ball easily), stop mixing; there should still be visible pieces of fat in the dough. Flatten the dough into a disk and wrap the two disks in plastic wrap or waxed paper. Refrigerate for 30 minutes or longer; this resting period allows the flour to absorb the water, making the dough easier to roll out.
When you’re "ready to roll," remove the dough from the fridge. Dough made with a combination of butter and shortening should rest for about 5 minutes at room temperature before rolling. If you've made the dough with all vegetable shortening, you’ll be able to work with it directly from the refrigerator. A dough made with all butter will need to warm slightly (10 to 15 minutes) before rolling, as butter becomes brittle when it’s refrigerated.
Roll the dough to the size needed (about 13" for a 9" pie). Fill and bake as directed in your recipe.