Toss zucchini, tomatoes, onion, mushrooms and dressing in large bowl until well coated.
Brush grill rack lightly with vegetable oil, or spray with cooking spray. Heat coals or gas grill for direct heat. Place vegetables on grill, using slotted spoon. Grill uncovered 4 to 6 inches from medium heat 4 to 5 minutes, turning once and brushing with remaining dressing in bowl, until crisp-tender. Refrigerate vegetables uncovered in single layer while making pasta.
Empty Pasta mix into 3-quart saucepan 2/3 full of boiling water. Gently boil uncovered 12 minutes, stirring occasionally.
Using same bowl with any remaining dressing, stir together Seasoning mix and cold water; set aside. Drain pasta; rinse with cool water. Shake to drain well. Cut grilled vegetables in half if desired; stir vegetables and pasta into seasoning mixture. Serve immediately, or refrigerate.