Preheat oven to 350 degrees F. Spray a 9 x 9 inch baking dish with nonstick cooking spray.
In a small saucepan, heat white sugar, cornstarch, and water over medium heat. Cook until thick, whisking occasionally.
Remove sauce from heat; stir in vanilla. Set aside.
In a large bowl, mix flour, oats, brown sugar, melted butter and cinnamon until crumbly.
Take 1/2 of the oatmeal crumble and press into the bottom of the prepared baking dish.
Sprinkle with diced rhubarb.
Pour prepared vanilla sauce over rhubarb.
Sprinkle remaining half of crumble over the top.
Bake for 1 hour or until bubbly and browned.
Serve as is or with ice cream, whipped cream or drizzled with half and half.