Beat brown sugar and margarine. Beat in egg. Combine ingredients and add milk alternately with buttermilk. Stir in vanilla and rhubarb. Pour into greased 9-13 inch pan. Combine topping ingredients to make crumbs. Sprinkle on top of batter and bake at 350 degrees until toothpick inserted in center comes out clean, 20-30 minutes.
Madagascar Vanilla (purchase at Chefs Catalog) Of all the vanilla's I've used this is my favorite. Once you've tried it you won't go back!