Preheat oven to 375 degrees F.
In a large bowl, stir together flour, quick oats and brown sugar. Add in melted butter. Take 1 1/2 cups of topping mixture and set aside. With the remaining mixture, press into the bottom and sides of a 9-inch pie pan. Set aside.
Place apples in a large bowl; set aside.
In a medium size sauce pan, add the sugar, cornstarch and water. Bring the mixture to a boil until thick. (It will thicken up very quickly, so make sure you are standing by that stove. Don't step away.) Pour mixture in with the apples.
Stir in vanilla, cinnamon and lemon juice. Mix well.
Pour filling into pie crust and top with remaining reserved topping mixture.
Bake for 35-45 minutes (mine baked for 33 minutes). If your pie starts to brown to much on top, place some foil over it.
Once the pie is done baking, place it on the stove for 5 minutes, then serve with ice cream, or by itself.