Trim fat from lamb; cut lamb into 3/4-inch pieces. Combine vinegar and next 6 ingredients in a heavy-duty, zip-top plastic bag. Add meat, green pepper, and onion; seal bag, and shake until meat and vegetables are coated. Marinate in refrigerator 8 hours, turning bag occasionally. Combine yogurt and next 4 ingredients. Cover and chill.
Remove meat and vegetables from marinade, reserving marinade. Place marinade in a saucepan; bring to a boil. Remove from heat; set aside. Thread meat, pepper, and onion onto 4 (15-inch) skewers. Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place kabobs on rack; grill, covered, 10 minutes or until meat is done, turning and basting often with marinade.
Cut 1/4 inch off top of each pita round, reserving tops for another use. Fill pitas evenly with lettuce and tomato. Spoon meat and vegetables into pitas. Top with yogurt mixture.