Add rhubarb, water, and sugar to a three-quart kettle. Scrape seeds from a vanilla bean and add to the mixture along with the bean.
Cook over medium-high heat, bringing to a simmer and stirring constantly to prevent sticking or burning, about 6-10 minutes. The rhubarb will soften and the water will evaporate, leaving a thick, jam-like filling.
Remove from heat and let cool.
Preheat oven to 350°F.
Line an 8x8 baking dish with an aluminum foil sling. Spray with nonstick spray and set aside.
Mix together all crumb topping ingredients except butter until incorporated.
Add the softened butter in pieces and incorporate it into the dry mixture using fingers (you can use a pastry cutter but fingers work very well). Mix until no large pieces of butter remain and the mixture becomes like wet sand in texture.
Lightly press 2 cups of the dry mixture into the bottom of a prepared pan.
Spoon over cooled jam mixture and spread evenly.
Evenly coat top with remaining crumb mixture. Make some large crumb chunks by pressing together crumb in hand (my favorite).
Place in preheated oven and bake until edges just start to bubble and crumb turns lightly brown, about 30-35 minutes.