Preheat to 350 F. Stir the rhubarb, sugar, flour and vanilla extract together in a bowl to evenly coat and then spread the mixture out in a buttered 13x9 inch baking dish.
In a large bowl add the flour, oats, brown sugar, butter, cinnamon and salt and work it until combined and crumbly (use your hands, a fork or a pastry cutter). Sprinkle evenly over the rhubarb.Bake on middle rack for about 40 minutes or until the topping is browned and the rhubarb is bubbling. Let cool about 10 minutes before serving to allow the rhubarb filling to soak up excess juices. Serve warm with a scoop of vanilla ice cream.