Combine rhubarb, sugar, tapioca, lemon peel, and orange peel in a bowl. Set aside. Place butter in a 1 cup glass measure. Cover with waxed paper and cook on Hi for 45 seconds, or until butter melts. Pour butter over bread cubes in small bowl. Add vanilla and mix lightly. In a 1 1/2 quart micro proof casserole, make alternate layers of rhubarb and bread cube mixture, ending with buttered bread cubes. Cover and cook on HI for 5 minutes, or until rhubarb is cooked Serve warm or chilled. Yield: 6 to 8 servings