Heat oven to 350°F. In medium bowl, combine cake mix and vanilla. Cut in 1/2 cup butter, using pastry blender (or pulling 2 table knives through mixture in opposite directions), until mixture is crumbly. Reserve 1 cup mixture for topping. In bottom of ungreased 8-inch square pan, press remaining crumb mixture. Bake 15 minutes. Meanwhile, stir walnuts, oats and melted butter into reserved crumb mixture.