Combine first 9 ingredients in a medium saucepan; bring to a boil. Reduce heat, and simmer, uncovered, 15 minutes. Combine zucchini and onion in a large bowl; pour hot vinegar mixture over vegetables. Cover and let stand at room temperature 3 hours. Chill 8 hours before serving.
Note: Store, covered, in refrigerator up to 1 month.