Preheat the oven to 375°.
Butter a 3-quart baking dish.
To make the crumble--mix the flour, oats, brown sugar, pecans, and salt together in a bowl.
Stir in the butter, then press the mixture together with your hands to form a few small clumps.
Put the topping in the freezer while you assemble the filling.
To make the rhubarb filling, rub the sugar and cornstarch together in a bowl, then add the rhubarb and vanilla and toss until evenly coated.
Transfer the rhubarb mixture into the prepared pan and scatter the crumble topping over the top.
Bake for about 45 minutes, or until the topping is golden and the filling bubbles up through the topping.
Cool for 20 minutes before serving.
Storage--the crisp is best served the day it is made, but any leftovers can be wrapped in plastic wrap and eaten for breakfast the next morning or kept at room temperature for 2-3 days.