1. Cut chicken into thin slices and place in bowl, sprinkle with salt and pepper.
2. Sprinkle on vinegar and chives.
3. In a small bowl, mix together yogurt and cucumber.
4. Split the pitas, making pockets. In each pita, spread 2 tablespoons yogurt-cucumber sauce.
5. Place 1/4 of the chicken mixture into each pocket; close the pockets, wrap in plastic wrap and chill in refrigerator overnight, cut in half to serve.