In a large bowl, cream together butter or margarine and sugar. Beat in egg and vanilla.
In another bowl, sift together 2 cups flour, soda, and salt. Add sifted ingredients alternately with vegan buttermilk to creamed mixture.
Toss chopped rhubarb with 1 tablespoon flour, and stir into batter. Spoon batter into buttered 9 x 13 inch pan, and smooth the surface.
Blend together 1/4 cup butter, cinnamon, and brown sugar; sprinkle evenly over batter.
Bake at 350 degrees F for 45 minutes.