Trim ends of yellow squash and zucchini; cut crosswise into 1-in. slices. Place slices in a large bowl; add potatoes. Pour oil over mixture, tossing to coat.
Cut turkey into 24 cubes; add to vegetables. Sprinkle with pepper and salt; toss again.
On 6 metal or soaked wooden skewers, alternately thread turkey cubes, squash, zucchini and potatoes. Grill, covered, over medium heat, turning occasionally, until turkey is no longer pink and vegetables are crisp-tender, 12-15 minutes. Serve on greens with croutons. Drizzle with vinaigrette.