Chicken: Place chicken between plastic wrap; pound with meat mallet or bottom of heavy saucepan to 1/4-inch (5 mm) thickness.
In bowl, combine vinegar, oil, garlic, salt and pepper. Add chicken and turn to coat. Cover and refrigerate for up to 24 hours.
Meanwhile, in medium saucepan, boil 1 1/2 cups low-sodium vegetable broth or water. Stir in couscous. Remove from heat. Cover and let stand for 5 minutes. Fluff with fork.
Grill vegetables on greased grill over medium heat; close lid and grill, turning, until tender, about 8 minutes. Cut into 1-inch (2.5 cm) pieces; add to couscous.
In small bowl, whisk together oil, vinegar, pepper, garlic and sugar. Add to couscous; toss to coat.
Reserving marinade, grill chicken on greased grill over medium heat; brush with some of the marinade.
Turn chicken and brush with any remaining marinade; grill until no longer pink inside, about 4 minutes.