Heat oven to 350 degrees F. Spray a 9"X9" glass baking dish with non-stick cooking spray. Set aside.
For the topping and crust: In a large bowl, combine the oats, brown sugar, flour and cinnamon. Add the margarine and combine ingredients, using a pastry blender or two forks, until coarse crumbs form (about the size of a pea).
Press half of the mixture into the bottom of prepared baking dish. Place the diced rhubarb on top of crust, spreading evenly. Set aside.
In a medium size saucepan, combine the sugar, cornstarch, water and vanilla. Cook over medium high heat, stirring constantly, about 10 minute, or until mixture thickens and is the consistency of cooked pudding. Set aside to cool.
When cornstarch mixture is cooled, pour over rhubarb in baking dish. Sprinkle remaining crumbs evenly over top.
Bake for 45-50 minutes or until hot and bubbly around edges and top is starting to brown. Serve warm with vanilla ice cream or whipped cream.