Prepare grill to medium-high heat.
Combine first 7 ingredients in a large bowl, tossing to coat. Remove vegetables from bowl, reserving vinegar mixture. Place the onion and bell pepper on grill rack coated with cooking spray; grill 7 minutes on each side or until tender. Grill zucchini 3 minutes on each side or until tender.
Hollow out bottom half of bread, leaving a 1/2-inch-thick shell; reserve torn bread for another use. Layer grilled vegetables, greens, cheese, and basil in bottom of loaf; drizzle reserved vinegar mixture on top. Cover with top of the bread; press lightly.
Place filled loaf on grill rack; grill 4 minutes on each side or until cheese melts. Cut into quarters.