04Cook tomatoes (still on vine, if using) in disposable roasting pan, covered with grill lid, carefully turning, until softened and just beginning to split, about 8 minutes. Drizzle balsamic vinegar over tomatoes, turning to coat, then cook, uncovered, until vinegar is reduced by about half, about 2 minutes. Remove from grill.
05Bring reserved marinade with any lamb juices accumulated on platter to a boil in a small heavy saucepan, covered.
06Drizzle lamb with marinade and serve with tomatoes.