In a bowl, combine flour, oats, 1 cup brown sugar, salt, cinnamon and butter. Pat about half of mixture onto the bottom of a greased 9-in. square baking pan. In a saucepan, combine cornstarch and remaining brown sugar; stir in water. Cook and stir until mixture comes to a boil and is thickened and clear. Remove from the heat; stir in vanilla. Add rhubarb and toss until coated. Spoon over crust; sprinkle with reserved flour mixture. Bake at 350° for 50 minutes or until bubbly.