1. Prepare grill to medium-high heat.
2. Combine first 3 ingredients in a large bowl. Sprinkle zucchini mixture with 1/4 teaspoon salt and 1/4 teaspoon pepper; toss gently to coat. Arrange vegetables in a single layer on a grill rack; grill 4 minutes on each side or until zucchini is tender and vegetables are well marked. Remove zucchini from grill; reduce grill heat to medium-low. Grill onion an additional 5 minutes or until tender. Combine zucchini, onion, and vinegar in a large bowl, tossing to coat. Sprinkle with remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, cheese, and mint.