Grease a 13x9x2-inch baking pan; set aside. For filling: In a medium saucepan, combine rhubarb, the 1 cup granulated sugar and water. Bring to boiling; reduce heat. Cover and simmer for 5 minutes. Meanwhile, in a small bowl combine the 1/2 cup granulated sugar and the 2 tablespoons flour. Stir into rhubarb mixture. Cook and stir about 1 minute more or till thick. Remove from heat; stir in vanilla. Set aside.
In a medium mixing bowl, combine the 1 1/2 cups flour, oats, brown sugar and baking soda. Using a pastry blender or 2 knives, cut in shortening till the mixture resembles coarse crumbs. Stir in 1/2 cup pecans or walnuts. Reserve 1 cup of the crumb mixture.
Press remaining crumb mixture into the bottom of prepared pan. Evenly spread rhubarb mixture on top. Sprinkle with reserved crumb mixture and additional chopped nuts, if you like. Bake in a 375° oven for 30 to 35 minutes or till the top is golden. Cool on a wire rack. Cut into bars. Makes 45 bars.