For marinade: In a 3-quart rectangular baking dish, whisk together vinegar, olive oil, garlic, oregano, and salt. Add zucchini and onion, stirring to coat. Marinate at room temperature for 10 minutes.
Meanwhile, lightly coat a grill pan or grill wok with cooking spray. For a charcoal grill, preheat grill pan on an uncovered grill directly over medium coals for 15 seconds. Using a slotted spoon, remove zucchini and onion from marinade and place in the grill pan. Reserve marinade. Grill vegetables for 5 to 6 minutes or just until tender and lightly brown, stirring occasionally. Stir in the tomatoes. Grill about 1 minute more or until tomatoes are heated through. (For a gas grill, preheat grill. Reduce heat to medium. Preheat grill pan as directed. Add vegetables as directed. Cover and grill as above.) Remove vegetables from the grill pan. Place on a serving platter. Drizzle reserved marinade over vegetables. Toss to combine.