Mix together flour, oatmeal, brown sugar, butter, and cinnamon. In a saucepan, cook sugar, cornstarch, water, and vanilla until thin and clear. Grease a 9-inch baking dish. Put 1/2 of the oatmeal/flour crumbly mixture into the bottom of baking dish and then add rhubarb. Pour the cornstarch/sugar mixture over the top of rhubarb and then top with the rest of the oatmeal/flour crumbly mixture. Bake 1 hour at 350 degrees.