Combine the flour, sugar, and salt in a medium bowl. Add the butter, and kit it into the flour using a pastry blender or two knives until evenly incorporated. Add the vinegar and 3 tablespoons of water, and stir with a fork until a shaggy dough forms. If the dough doesn't clump together nicely and there are dry bits at the bottom of the bowl, add up to 2 more tablespoons of water.
Press the crust into the bottom of an ungreased 9x5-inch loaf pan. Bake for 10 minutes and remove the pan from the oven. Then, turn the oven temperature down to 350 degrees.
Make the filling:
Sir together all the filling ingredients. Pour over the parbaked crust. Bake for 40 minutes. The pie bars are done when the top is nicely browned and dry.
Let cool completely in the pan, and chill overnight for the prettiest slices. Serve chilled or warm, whichever you like.
Notes
While the instructions call for an ungreased baking pan, I lined my loaf pan with a "sling" of parchment paper across the bottom for easy removal of the bars once cooled.