Mix flour, oatmeal, brown sugar and butter until crumbly. Press 1/2 into greased 9" pan. Add rhubbard, cut in 1/2" pieces. Combine sugar, cornstarch, water and vanilla; cook till thick and clear. Pour sauce over rhubarb. Top with other half crumb mixture. Bake at 350F for 45 minutes. [Flanagan Parent-Teacher Club Cookbook GEnie/D.WEISSGERBE AOL/Rik W]