1) To make the crust: Combine the flour, buttermilk powder, salt, and baking powder, then mix in the butter and shortening till crumbly, leaving some pea-sized lumps.
2) Mix the vinegar with 4 tablespoons of the water. Sprinkle onto the dry ingredients, stir, and squeeze the dough together; if it's not totally cohesive, add an additional tablespoon of water (enough to make the dough stick together nicely, without crumbling).
3) Shape the dough into a flattened rectangle, wrap it in plastic wrap, and refrigerate for 30 minutes.
4) To make the filling: Heat the butter till melted, then stir in the flour. Gradually pour in the stock, whisking constantly. Cook and stir the sauce over medium heat till it comes to a boil, then reduce the heat and simmer for 5 minutes.
5) Stir in the chicken and salt. Add ground black pepper, additional salt to taste, and the vegetables. Spoon the filling into a 9" x 13" (2- to 3-quart, or similar size) deep casserole dish.
6) Roll the crust out slightly larger than the dish. Place it on top of the filling; cut several vent holes, and use any scraps of dough to decorate.
7) Bake the pie in a preheated 375"F oven for 50 to 60 minutes, until the crust is golden brown and the filling is bubbly.