Combine the flour, oats, brown sugar, cinnamon, and melted butter in a bowl; stir to form the crumbs.
Pat half the crumb mixture into a lightly buttered 8 or 9-inch square baking pan.
Cut the rhubarb into 1/4-inch slices and arrange it on the crust
Combine the sugar and cornstarch in a saucepan and whisk to blend. Gradually add the water, stirring until smooth and well blended. Place the pan over medium low heat and cook, stirring constantly, until thickened. Add the vanilla and pour over the rhubarb.
Top the rhubarb with the remaining crumbs.
Bake the crisp for 35 to 40 minutes or until the rhubarb is tender.