Beat sugar and butter at medium speed with an electric mixer until creamy. Add eggs, one at a time, beating until yellow disappears after each addition. Beat in lemon juice and vanilla. Combine flour and baking soda in a small bowl; add to sugar mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at medium speed just until blended after each addition. Spoon batter into paper-lined muffin cups, filling 2⁄3 full. Bake at 350 degrees for 18 to 22 minutes or until a toothpick inserted in center comes out clean. Cool in pans on a wire rack 10 minutes. Remove cupcakes from pans to wire rack; cool 45 minutes or until completely cool. Spread frosting on cupcakes. Immediately decorate with desired red, white and blue sprinkles. Place cupcakes on plate in a flag pattern. Makes 2 dozen.