Preheat oven to 180°C (350°F).
Prepare the rhubarb filling: In a large bowl, combine chopped rhubarb, granulated sugar, cornstarch, and vanilla. Toss to coat evenly. Transfer to a greased 8x8-inch baking dish.
Make the crisp topping: In another bowl, mix oats, flour, brown sugar, cinnamon, and a pinch of salt. Add cold cubed butter and mix with your fingers or a pastry cutter until crumbly.
Assemble: Sprinkle the oat mixture evenly over the rhubarb.
Bake for 40 minutes, or until the top is golden and the filling is bubbling.
Rest: Let cool for 10 minutes before serving.