First, cut up the rhubarb and the zucchini and put them in a medium bowl for later. I left the peel on the zucchini. It added some color. Feel free to peel it if you choose.
Mix the flour, oatmeal, butter, cinnamon, and brown sugar until crumbly. Press half of the crumb mixture into a greased 8x8 inch or 9x9 inch baking dish. Cover with the rhubarb and zucchini.
Combine the cornstarch, sugar, water, and vanilla and place in a medium size sauce pan. Cook until thick and clear. Pour over the rhubarb and zucchini and top with the remaining half of the crumb mixture.