1) To make the crust: Whisk together all of the dry ingredients.
2) Work in the shortening and then the butter until they're well combined with the dry ingredients.
3) Sprinkle the vinegar and 3 tablespoons of the water over the dough, and toss with a fork. Once the dough becomes cohesive stop mixing, only adding water if the dough doesn't easily hold together when squeezed.
4) Flatten the dough into a disk, wrap in plastic, and refrigerate for at least 1 hour.
5) Preheat the oven to 325°F and place the oven rack in its lowest position.
6) Roll the dough into a 12" to 13" circle and settle it gently into a 9" pie pan. Flute or crimp the edge of the crust as desired, then place in the refrigerator while you make the filling.
7) To make the filling: Beat the eggs slightly and add the sugar, flour, and salt.
8) Add the melted butter and mix well. Mix in the buttermilk and vanilla extract. Pour the filling into the unbaked crust.
9) Bake the pie for 20 minutes on the bottom rack of the oven, to allow the bottom to brown.
10) Move the pie to the middle of the oven, and continue to bake for another 40 minutes, until the custard is set and a very thin crust has formed on top.
11) Remove the pie from the oven and place on a rack. Serve warm, or cool to room temperature, then cover and refrigerate.
Yield: one 9" pie.