Place rhubarb, sugar, water, and vanilla in a small saucepan.
Bring to the boil over medium-high heat, then reduce heat and simmer for 10 minutes until rhubarb is soft, but stil chunky.
Crumble
Add all the dry ingredients together in a bowl. Add the butter and rub the dry ingredients and butter together with your fingtertips until it resembles chunky breadcrumbs or a crumble-like texture.
Assembly & Cooking
Preheat oven to 180C/350F.
Pour the rhubarb into a medium-sized oven proof dish.
Top with the crumble mixture.
Bake for 25-35 minutes, or until the crumble is browned on top.