Place the chicken breasts in a shallow dish or resealable bag and pour in half of the vinaigrette. Turn to coat and marinate for 15–30 minutes while the grill heats.
Remove the chicken from the marinade and discard the used liquid. Place the chicken on the grill grates and cook for about 6–8 minutes per side, or until the internal temperature reaches 165°F.
Transfer the chicken to a cutting board and let it rest for 5–10 minutes before chopping into bite-sized pieces.
In a large bowl, combine the Napa cabbage, bell pepper, onion, cucumber, tomatoes, and feta.
Add the chopped chicken to the salad. Drizzle with the remaining vinaigrette and toss until everything is evenly coated.
Top with the crispy onion topping right before serving for extra crunch.