Add water to a Dutch oven to a depth of 2 inches; bring to a boil. Add leeks; cover and simmer 5 minutes. Add zucchini, squash, and sweet red pepper; cover and simmer 5 minutes. Drain well; pat dry with paper towels.
Place flour in a bowl; cut in margarine with a pastry blender until mixture resembles coarse meal. Sprinkle cold water, 1 tablespoon at a time, evenly over surface of mixture; stir with a fork until dry ingredients are moistened. Shape into a ball.
Place dough between 2 sheets of heavy-duty plastic wrap, and gently press into a 4-inch circle. Chill 20 minutes. Roll into a 12-inch circle. Freeze 5 minutes. Remove top sheet of plastic wrap. Invert and fit pastry into a 9-inch quiche dish or pieplate; remove remaining sheet of plastic. Fold edges of pastry under, and flute; seal to edge of pieplate. Bake pastry shell at 450° for 5 minutes. Spoon vegetable mixture into pastry.
Combine eggs and next 5 ingredients; stir well with a wire whisk. Pour over vegetable mixture. Bake at 375° for 30 minutes; sprinkle with cheese. Bake 5 additional minutes or until cheese melts. Let stand 5 minutes before serving.