Preheat oven to 350°. Mix flour, brown sugar, oats and butter; reserve 3/4 cup mixture for topping. Press remaining mixture onto bottom and up sides of an ungreased 9-in. pie plate. , In a large saucepan, mix sugar, cornstarch and water until smooth; bring to a boil. Cook and stir until thickened, about 2 minutes. Remove from heat; stir in apples and vanilla. Pour into crust. Crumble topping over filling., Bake until crust is golden brown and filling is bubbly, 40-45 minutes. Cover edges with foil during the last 20 minutes to prevent overbrowning if necessary. Cool on a wire rack. If desired, serve with ice cream and caramel.