If using frozen pie crust, remove from freezer to thaw in refrigerator.
Chop apples into 1/4 inch slices. Combine with remaining apple filling ingredients (brown sugar, vanilla, salt, cinnamon, cornstarch, apple cider vinegar).
Mix apple mixture and set aside.
Make crumble topping by mixing together flour, oats, walnuts, brown sugar, cinnamon, salt and vegetable oil. Set aside.
Remove pie crust and roll out to pie dish size.
Fill in pie crust with apples in an even layer.
Top pie evenly with crumble mixture.
Bake at 350F for 45 minutes. If top is browning too quickly, cover with foil.
Allow to rest 10 minutes after baking before slicing. Enjoy!