Place corn and potatoes on a jelly-roll pan. Drizzle vegetables with 1 tablespoon oil; toss to coat. Back at 425 degrees for 30 minutes or until potatoes are tender. Place mixture in a large bowl. Combine tarragon and next 4 ingredients (through pepper) in a small bowl, sirring with a whisk. Gradually add remaining 1 1/2 tablespoons oil, stirring constantly with a whisk. Drizzle potato mixture with dressing; toss gently to coat.
Heat a large skillet over medium heat. Coat pan with cooking spray. Add onion and zucchini to pan; cook 4 minutes or until lightly browned, stirring occasionally. Add zucchini mixture and tomatoes to potato mixture; toss gently to combine.