This is the perfect summer salad!
For the dressing, whisk together the mayonnaise, buttermilk, chopped dill and pepper. Add the green onions. Can be made 1 day ahead.
Arrange chicken in a heavy skillet or pot. Add wine and dill stalks, and season with salt and pepper. Add water to cover chicken. Simmer until chicken is just cooked through, turning once, about 12 minutes. Transfer to plate and cool.
Cut chicken into 1/2" pieces. In a large bowl, combine chicken, grapes, celery and dressing. Season with salt and pepper. Refrigerate at least 1/2 hour.