To prepare crust, lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, powdered sugar, and 1/4 teaspoon salt in a medium bowl; cut in shortening with a pastry blender or 2 knives until mixture resembles coarse meal. Combine water and vinegar. Sprinkle surface of flour mixture with water mixture, 1 tablespoon at a time; toss with a fork until moist. Gently press mixture into a 4-inch circle on plastic wrap, and cover. Chill for 1 hour.
Preheat oven to 400°.
Slightly overlap 2 (15-inch) sheets of plastic wrap on a slightly damp surface. Unwrap chilled dough, and place on plastic wrap. Cover with 2 additional (15-inch) sheets of overlapping plastic wrap. Roll dough, still covered, into a 12-inch circle. Place dough in freezer 10 minutes or until plastic wrap can be easily removed.
Let stand 1 minute. Remove 2 sheets of plastic wrap. Fit dough, plastic wrap side up, into a 9-inch pie plate; remove remaining plastic wrap. Press the dough against bottom and sides of pie plate. Fold edges under, and flute. Freeze 10 minutes.
Line bottom of dough with a piece of foil; arrange pie weights or dried beans on foil. Bake at 400° for 20 minutes or until pastry edge is lightly browned. Remove pie weights and foil; bake an additional 8 minutes or until done. Cool completely on a wire rack.
To prepare filling, melt butter in a large, heavy saucepan over medium heat. Add brown sugar; cook 2 minutes or until mixture resembles wet sand, stirring constantly with a wooden spoon. Add 1/4 cup evaporated milk, 1 tablespoon at a time, stirring after each addition. Bring to a boil, stirring constantly; cook 30 seconds. Remove from heat.
Combine remaining 3/4 cup evaporated milk, cornstarch, 1/8 teaspoon salt, and egg yolks in a medium bowl, stirring with a whisk. Heat 2% milk in a small saucepan over medium heat to 180° or until tiny bubbles form around edge (do not boil), stirring frequently. Gradually add hot milk to cornstarch mixture, stirring constantly with a wire whisk.
Gradually add hot milk mixture to brown sugar mixture, stirring with a whisk. Place mixture in a pan; cook over medium heat 11 minutes or until thick, stirring constantly with a wooden spoon. Cook an additional 2 minutes; remove from heat. Stir in vanilla. Place pan in a large ice-filled bowl for 20 minutes or until mixture comes to room temperature, stirring occasionally.
Remove pan from ice. Spoon mixture into the prepared crust. Cover and refrigerate for 8 hours or overnight. Spread whipped topping evenly over filling just before serving.