With a sharp paring knife, remove a thin slice from the top and bottom of each grapefruit, then remove the skin of the fruit in long strips. Set aside. Cutting between the membrane, cut the grapefruit into sections and set aside. Place the sugar and wine vinegar in a heavy-bottomed saucepan. Cook over medium high heat until the vinegar has evaporated and the sugar has caramelized lightly. Carefully add the stock and bring the mixture to a boil. Simmer at low boil for 5 minutes. Add grapefruit skins to the mixture and simmer 3 minutes longer. Strain the mixture into a clean saucepan and simmer until reduced enough to coat a spoon lightly. Adjust seasoning with salt and pepper and keep it warm. Transfer the duck to a work surface. Run a sharp knife under the wishbone at the front of the duck's breast, then carefully slip the knife between each breast half and carcass. Remove the breasts, and then take off the legs and thighs in one piece. Lightly coat a warm serving plate with the sauce. Place the duck over the sauce and arrange the grapefruit sections, overlapping, on top. Garnish with blanched vegetables as desired, and serve, passing the rest of the sauce around separately.