In a small bowl mix together 2 tablespoons of all-purpose flour, and ½ cup granulated sugar; set aside.
In a medium sauce pan mix together the rhubarb, one cup granulated sugar, and water. Cook over medium high heat until mixture comes to a boil, reduce heat to simmer, cover, and cook for 5 minutes. Stir the sugar flour mixture into the rhubarb and continue cooking until thickened; stir in vanilla and set aside.
In a large bowl mix together the flour, oats, brown sugar, and baking soda. Using a pastry blender or fork cut the shortening into the flour mixture to form a crumbly dough; reserve 1 cup of the dough for the topping.
Lightly spray a 13 x 9 x 2-inch pan with cooking spray, and press the dough into the bottom of the pan. Spread the rhubarb over the crust and then sprinkle the remaining dough over the top.
Bake for 30 – 35 minutes or until golden brown – cool before serving.