On 8 12-inch metal or bamboo skewers, alternately thread chicken, mushrooms, green onions and plums.
In a small bowl stir together vinegar and oil; brush on kabobs.
For a charcoal grill, place skewers on the rack of an uncovered grill directly over medium coals for 12 to 15 minutes or until chicken is cooked through, with no pink remaining, turning once halfway through grilling time. (For a gas grill, preheat grill. Reduce heat to medium. Place skewers on a grill rack over heat. Cover and grill as above.)
Transfer skewers to serving platter; sprinkle with salt and freshly ground pepper.