Method:
Put vinegar, oil, rosemary, garlic, pepper, sugar, and salt into a wide, shallow dish and stir together to make a marinade. Remove netting or string from leg of lamb and lay out flat in dish. Turn to coat with marinade, then cover dish and refrigerate, turning several times, for 6 to 24 hours.
Start fire, using the equivalent of 2 gallons of charcoal. While coals are heating, roll and tie the leg of lamb. When coals are glowing, spread the coals into 2 piles on opposite edges of grill. (For extra flavor, toss mesquite chips, soaked in water for about 15 minutes, over hot coals prior to cooking.)
Place cooking grill over coals and top with a piece of foil in the center of the grill to catch drippings. Place lamb on foil and cook, covered but with air vents open, until desired doneness, 75 to 90 minutes. Remove from grill, set aside to let rest for 10 minutes, then slice lamb and serve.