In a large bowl, combine flour, brown sugar, baking soda and salt. Combine oil, egg, buttermilk and vanilla; stir into dry ingredients just until moistened. Fold in rhubarb and walnuts. Fill greased or paper-lined muffin cups two-thirds full. Combine topping ingredients; sprinkle over muffins. Bake at 375 degrees F for 20-25 minute or until muffins test done. Cool 10 minutes before removing to a wire rack.