1. In a large bowl, cream margarine, sugar, egg and vanilla.
2. Combine flour, soda and salt.
3. Add dry ingredients alternately with buttermilk to the creamed mixture. Start and end with the dry ingredients.
4. Toss rhubarb with flour, and carefully stir into the batter.
5. Pour into a greased and floured 9x12 pan, and smooth the surface.
Bake at 350° for 45 minutes.