Prepare the sauce by mixing together the sesame oil, vinegar, soy sauce, and sugar.  Heat the vegetable oil in a wok or large skillet.  Add the greens and stir-fry until just tender, but not overcooked.  Pour off any excess oil or liquid.  Toss the greens with the sauce in a serving bowl.  Sprinkle the sesame seeds over the greens and serve.  We like this dish best stir-fried, but if you wish to reduce your consumption of oil, it may be prepared by cooking the cooking the spinach in ½ cup water until limp, draining it, and then proceeding as above.