Rinse and trim the ends off the rhubarb stalks. Slice them in 1 inch pieces and put in a heavy pot. Toss with the sugar, lemon juice, and water. Stirring constantly, bring the mix to a boil and cook until the rhubarb has broken down and the mixture is thick. Add a little more water if needed as it cooks if it gets too thick. This will take about 20 minutes or so. You can puree it if you want a smooth texture, I used an immersion blender and pulsed a few times to get any larger lumps, but some texture is fine. Stir in the vanilla and set aside to cool.
To make the crust, put the dry ingredients in a food processor and pulse to combine. Add in the butter and pulse until the mix has a texture of coarse crumbs.
Reserve 1 1/2 cups of the mixture and press the rest into a buttered 9x13 baking pan. Pat down the crust and level it out.
Spread the filling evenly over the crust, and then crumble the remaining dough over the filling.