Pour balsamic vinegar in a small saucepan. Simmer to reduce to 3 tbsp., cool vinegar. Add cooled vinegar to yogurt with oil, garlic and parsley. Divide mixture in half. Place prepared zucchini, eggplant and onion on a sheet pan and brush with 1/2 of the yogurt mixture. On a lightly oiled preheated grill, cook vegetables until golden and brown (3-4 minutes). Place cooked vegetables on a platter and sprinkle with red peppers. Drizzle remaining dressing over vegetables and serve.