Combine butter, brown sugar, flour, oats and cinnamon in a medium mixing bowl. Stir until well combined. Set aside about 1 cup of oat mixture for topping.
For the crust, press rest of oat mixture into bottom and sides of a 9-inch pie plate
Stir together cornstarch, cinnamon and sugar in the bottom of a large stock pot. Whisk in cold water. Bring to boil and cook until thickened, stirring constantly. Remove from heat. Stir in apples and vanilla.
Dump apple filling onto oatmeal crust. Press on remaining oatmeal topping. Press it firmly into the filling to keep it from sliding off when cooking and slicing.