Mix together flour, oats, brown sugar, spices, and melted butter. Press half of the mixture into the bottom of a 9-inch square baking pan. Arrange the rhubarb and apples evenly on top. Cook the white sugar, water, cornstarch, and vanilla until thick; pour over the rhubarb and apples. Sprinkle the remaining oatmeal mixture on top. Bake at 350 degrees F for 1 hour. Serve warm with ice cream or whipped cream.