In a small bowl, whisk together balsamic vinegar, Italian dressing and garlic. Reserve 1/3 cup marinade. Combine remaining marinade with chicken breast pieces in a bowl and marinate for at least 15 minutes (up to 12 hours).
1 lb chicken breast, 2/3 cup balsamic vinegar, 2/3 cup Italian dressing, 1 clove garlic
Meanwhile, heat grill to medium heat.
Toss tomatoes, mushrooms, peppers and zucchini with olive oil. Season with salt and pepper. Add tomatoes and peppers to grill basket and place on grill. Cook, shaking frequently, for 5 minutes. Add mushrooms and zucchini to grill basket. Brush with some of reserved marinade and cook for another 5-7 minutes.
1 pint grape tomatoes, 1 pint white mushrooms, 2 bell peppers, 2 zucchini, 1 Tbsp olive oil, Salt and pepper
Remove chicken from marinade and add chicken to separate grill basket (or wait for vegetables to cook). Cook for 7-9 minutes, or until fully cooked.
Toss together chicken, grilled vegetables and orzo. Drizzle with 2 Tbsp reserved marinade. Serve warm or cold.
1 1/2 cup cooked orzo
Notes
The chicken can be marinated overnight. Fifteen minutes is the minimum recommended marinade time.
You can substitute boneless, skinless chicken thighs but it may impact the cooking times.