Preheat oven to 350° F and grease a 9-inch pie pan. Combine flour, brown sugar, melted butter and oats. Remove 1 cup of the mixture for the topping. Press remaining mixture into a 9-inch pie pan. For the filling, combine granulated sugar, corn starch and water in a saucepan until smooth, then bring to a boil. Cook and stir for 1 minute or until thickened. Remove from heat, stir in apples, vanilla and cinnamon. Pour filling into prepared crust. Top filling with remaining oat mixture and sprinkle with cinnamon to taste. Bake at 350° F for 40 minutes or until crust starts turning to a golden brown and filling is bubbling along the sides.