Preheat the oven to 350°F (175°C).

In a mixing bowl, combine the chopped rhubarb, granulated sugar, cornstarch, and vanilla extract.

Transfer the rhubarb mixture into a 9-inch baking dish and spread it evenly.

In another bowl, mix the flour, oats, brown sugar, and salt to create the crumble topping.

Incorporate the softened butter into the dry ingredients until crumbly.

Spread the crumble topping over the rhubarb mixture.

Bake for 30-35 minutes until the topping is golden brown and the rhubarb is bubbling.

Let the crumble cool for about 10 minutes before serving.