1. Cut bell peppers in half, and remove seeds. Slice squash and zucchini into 1/4-inch-thick rounds. Place peppers, squash, zucchini, and mushrooms in a large shallow dish or large zip-top plastic bag.
2. Whisk together vinaigrette, lemon pepper, and salt until blended. Pour over vegetables. Cover and chill at least 30 minutes. Remove vegetables from marinade, reserving marinade.
3. Grill vegetables, covered with grill lid, over medium-high heat (350° to 400°), basting with reserved marinade. Grill peppers 7 minutes on each side or until tender; grill squash and zucchini 3 minutes on each side or until tender. Grill mushrooms 5 minutes or until tender. Grill tomatoes in a grill wok or metal basket 2 to 3 minutes or until skins begin to split.