In a mixing bowl, combine brown sugar, applesauce and oil. Add egg, mix well. Combine vinegar and milk in the measuring cup, then add to mixture. Add vanilla. Combine the flour, baking soda and salt; stir into brown sugar mixture just until combined. Fold in the rhubarb. (If you like, you can also fold in 1/2 c of walnuts or pecans at this time.) Pour into two greased 8-in. x 4-in. x 2-in. loaf pans.
For topping, in a bowl, combine sugar, cinnamon and butter until crumbly; sprinkle over batter.
Bake at 350 degrees for 60 minutes or until a toothpick inserted near the center comes out clean.