Preheat oven to 325 degrees. Mix the first 4 ingredients and put 3/4 of the mixture in the bottom of a 9 x 13-inch baking dish. Save remainder of mixture for the topping. Put cut up rhubarb on top of brown sugar/oatmeal mixture. In saucepan, mix water, sugar and cornstarch and cook on medium heat, stirring constantly until it comes to a boil, thickens and turns clear. Remove from stove, add vanilla and pour over the rhubarb. Top with the remaining crumbs. Bake 60 minutes.
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