Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: In a large bowl, combine the chopped rhubarb, granulated sugar, all-purpose flour, and vanilla extract. Toss gently until the rhubarb is evenly coated. Pour this mixture into your prepared baking dish and spread it evenly.
Step 3: In a separate medium bowl, combine the rolled oats, brown sugar, all-purpose flour, ground cinnamon, and a pinch of salt. Whisk these dry ingredients together thoroughly.
Step 4: Add the cold, cubed unsalted butter to the dry topping mixture. Using your fingertips, a pastry blender, or a fork, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
Step 5: Evenly sprinkle the crisp topping over the rhubarb filling in the baking dish. Make sure to cover the entire surface.
Step 6: Place the baking dish on a baking sheet and bake in the preheated oven for 45-55 minutes, or until the topping is golden brown and the rhubarb filling is bubbling merrily. If the topping starts to brown too quickly, lightly tent it with aluminum foil.
Step 7: Once baked, remove the rhubarb crisp from the oven and let it cool on a wire rack for at least 15-20 minutes before serving. This allows the filling to set. Serve warm.