In a large mixing bowl, cream butter and sugar until light and fluffy. Add the eggs and vanilla and beat well. Sift together the dry ingredients. Add to butter mixture, alternately with juice and milk (do not combine the juice and milk) Pour batter into two greased and floured 8 inch round cake tins. Bake at 350 F for 30-35 minutes or until a cake tester comes out clean. Cool ten minutes in pan and turn out onto a rack to cool completely. Fill and ice with fluffly grapefruit frosting. Decorate with fresh grapefruit sections just prior to serving.