Combine oatmeal, flour, brown sugar, ¼ cup white sugar, baking soda, salt, vanilla and shortening until crumbly.
Pack a thin layer of the crumb mixture into the bottom of an 8- or 9-inch cake pan.
Add chopped rhubarb and sprinkle with ¾ cup white sugar.
Cover with the remaining crumb mixture.
Bake at 375 degrees for 40 minutes.