Preheat oven to 350º F. Line a 9×13 pan with foil (or parchment paper), leaving extra hanging over the ends. Spray with cooking spray.
(This step is just to help with getting the dessert out of the pan for easy cutting, you can just skip this step and spray the pan directly if you're going to cut the bars directly in the pan).
In a large mixing bowl, beat together the butter and sugar until light & fluffy, about 2-3 minutes.
Add vanilla, mix until combined. Add flour and mix until combined and dough is crumbly.
Press dough evenly into bottom of prepared pan.
Bake for 12-15 minutes, until dough is slightly puffed. Remove from oven and set aside, maintain oven temperature.
While crust is baking prepare the apples & topping.
In a large bowl, combine apples, flour and sugar. Mix until apples are thoroughly coated.
Arrange apples on baked crust.
In a medium bowl, mix together first five ingredients. With a pastry blender, cut in butter until mixture is crumbly.
Spread topping over apples in an even layer.
Bake uncovered for 45 minutes, or until topping is golden and apple mixture is bubbly.
Let cool, lift apple crisp using the extra foil at the ends as handles, and cut into bars.