Mix flour, brown sugar, oats, butter, and cinnamon together in a bowl until crumbly; press into the bottom of a 9-inch square baking dish. Spread rhubarb in an even layer over the crust.
Stir water, sugar, cornstarch, and vanilla extract together in a saucepan over medium low heat; cook, stirring occasionally, until thick and clear, 5 to 10 minutes. Drizzle sauce over rhubarb.
Bake in preheated oven until topping is set, about 1 hour.
