Preheat the oven to 350° F.
Make the crumb topping. Mix the flour, brown sugar, old-fashioned oats, and ground cinnamon in a medium bowl.
Add the butter and cut in with your fingers until the mixture resembles large coarse crumbs.
Make the filling. In a large bowl, mix the apples, cranberry sauce, sugar, vanilla, and cornstarch until well mixed.
Place the pie crust in a pie dish, and pour the apple filling into the shell. Mound it a little in the middle.
Sprinkle the crumb topping over the apples. There will be a lot. Pack it down a bit, if necessary.
Place the pie on a baking sheet, and place in the oven. Bake for about an hour and 10 minutes, or until the apples are tender and the crust and top are golden brown.
Transfer to a wire rack and cool for at least 15 minutes.
Serve with vanilla ice cream.