In a mixing bowl, combine brown sugar, flour, oats, butter, and cinnamon; mix together until crumbly.

Press half this mixture into a buttered 16" square baking dish, top with sliced rhubarb.

In a saucepan, combine granulated sugar, cornstarch, the water, and the vanilla.

Cook together until clear, pour over rhubarb. Do not need to use the whole mixture, the rhubarb releases juice when cooking.

Top rhubarb with remaining crumb mixture and bake at 350 degrees for 45-55 (50 worked well)