Preheat oven to 325 degrees. In a large mixing bowl, combine sugar, oil, egg, sweet potatoes and vanilla. In a separate bowl, mix dry ingredients (except walnuts) together well and add to wet ingredients. Stir until just combined. Fold in walnuts. Pour into 2 standard (9 x 5) loaf pans. Bake for 75 minutes or until thin knife inserted in center comes out clean.