Preheat oven to 350°F. Lightly coat a 12-cup muffin pan with cooking spray.

Whisk together all-purpose flour, whole wheat flour, baking soda, and salt in a medium bowl. Combine sugar and oil in a separate bowl, and beat at medium speed with an electric mixer until smooth. Add egg, and beat until light in color. Add flour mixture alternately with banana, mixing just until blended after each addition, beginning and ending with flour mixture. Stir in vanilla extract. Scoop batter evenly into prepared pan. 

Bake 18 to 20 minutes or until a toothpick inserted into the center of a muffin comes out clean. Cool in pan 5 minutes. Transfer muffins to a wire rack and cool completely.