In a medium bowl, combine chicken, ¼ cup vinaigrette, lemon zest and oregano; set aside.
Meanwhile, in a large salad bowl, toss lettuce with the remaining ¼ cup vinaigrette. Spoon 1½ cups lettuce into each of four shallow bowls. Top each with about ⅓ cup cucumber, ¼ cup tomatoes, 3 tablespoons sweet pepper, and 2 tablespoons onion. Add chicken mixture to the center of each. Sprinkle with 2 tablespoons feta and 1 tablespoon olives. If desired, serve with lemon wedges.