In mixing bowl, combine: brown sugar, flour, oats, butter and cinnamon. Mix together until crumbly. Press half of the brown sugar and oats mixture into a buttered 8-inch square baking dish. Top with the sliced rhubarb. In a saucepan combine: 1 cup granulated sugar, cornstarch, water, and vanilla. Cook together until clear, then pour over rhubarb. Top rhubarb with remaining crumb mixture. Bake at 350° for 45 to 55 minutes.