Preheat oven to 375 degrees. Lightly grease 12 muffin cups, or line with muffin papers.
In a large bowl, beat together oil and sugar. Beat in bananas, vanilla and egg. Add flour, baking soda, baking powder, salt and cinnamon. Mix together just until moistened. Spoon batter into prepared muffin cups.
In a small bowl, mix together brown sugar, 2 tbsp. flour, cinnamon and nutmeg. Cut in 1 tablespoon butter until mixture resembles coarse cornmeal. Sprinkle topping over muffins.
Bake in preheated oven for 18 to 20 minutes, until a toothpick inserted into center of a muffin comes out clean.