For the filling: Preheat the oven to 375 degrees F. Lightly butter an 8-inch square baking dish.
Toss together the rhubarb, granulated sugar, flour, cinnamon, vanilla, lemon and salt in a large bowl until the rhubarb is well coated. Fill the prepared baking dish and set aside.
For the crisp topping: Stir together the flour, brown sugar, butter, oats and salt in a medium bowl until crumbly. Spoon all of the crumble topping evenly over the prepared rhubarb and bake until golden and bubbling at the edges, about 35 minutes. Let cool slightly. Serve with ice cream, if using.