Cut eggplant into 1-inch thick slices. in colander, layer eggplant, sprinkling each layer lightly with salt. Let drain for 30 minutes. Meanwhile, grill red and yellow peppers on greased grill over medium-high heat, turning often, for 10 minutes or until roasted. Let cool. Peel, seed and cut into 1-inch squares. Place in large bowl.
Cut zucchini and onion into 1/2-inch thick slices; set aside separately. Trim ends of muchroom stems. Rinse eggplant under cold water; pat thoroughly dry.
Lightly brush vegetables with oil. Grill mushrooms and onion, turning occasionally, for 8-10 minutes. Grill zucchini and eggplant for 10 minutes or until tender-crisp. halve zucchini, quarter onions and cube eggplant slices; add to peppers along with mushrooms.
Dressing: in bowl, whisk together vinegar, mustard, garlic, salt and pepper; whisk in basil, then oil in steady stream. Pour over vegetables, tossing gently to coat. Refrigerate for at least 4 hours or up to 8 hours. Bring to room temperature before serving. Garnish with tomato and olives.