Preheat oven to 400 degrees F (200 degrees C). Grease muffin cups or line with paper muffin liners.
In a medium bowl, mix buttermilk with wheat bran. In another bowl, beat together egg, sugar, oil, banana, and vanilla.
In a large bowl, stir together flour, oat bran, baking soda, and baking powder. Stir buttermilk mixture and egg mixture into flour mixture until flour is just moistened. Stir in berries and walnuts. Spoon batter into muffin cups.
Bake in preheated oven for 20 minutes, or until done.