Preheat the oven to 350°F (175°C).
In a large bowl, combine the four, oats, brown sugar, cinnamon, and melted butter. Mix until crumbly.
In a saucepan, combine the granulated sugar, water, cornstarch, and vanilla extract. Cook over medium heat until thick and clear.
In a greased 9-inch square baking dish, press half of the crumb mixture, then add the diced rhubarb, and pour the sugar mixture over the rhubarb. Cover with the remaining crumb mixture.
Bake in the preheated oven for 1 hour or until the topping is golden brown and the rhubarb is tender.
Cut into squares and serve with whipped cream or ice cream.