1. Soak skewers in water 30 minutes.
2. While skewers soak, combine vinegar and next 4 ingredients in a small bowl, stirring with a whisk. Cut each onion half into 6 wedges.
3. Thread 2 pieces of chicken, 2 zucchini slices, and 1 onion wedge onto each skewer. Place kebabs in a shallow dish; add marinade. Cover and chill 1 hour, turning occasionally.
4. Prepare grill.
5. Remove kebabs from dish, reserving marinade. Coat kebabs with cooking spray. Place on grill rack. Cover and grill 14 to 16 minutes or until chicken is done, turning twice.
6. Bring reserved marinade to a boil in medium saucepan; boil 1 minute. Remove from heat; drizzle over skewers.