In nonstick fry pan, pour olive oil. Heat about 2 minutes on medium temperature. Add chicken and cook about 6 minutes. Turn chicken, add onion and cook about 6 minutes more or until onion is clear and chicken is tender. Remove from heat. Cut chicken into thin slices across grain of breast; place in bowl. Add sautéed onion; sprinkle with salt and pepper, mixing carefully. Add vinegar and chives; mix well. In small bowl, mix together yogurt and cucumber. On each piece of pita bread, spread 2 tablespoons yogurt cucumber sauce. Then place 1/4 of the chicken mixture on pita bread; fold in half, wrap in plastic wrap and chill in refrigerator for at least 2 hours. Repeat for remaining sandwiches. For picnic, place in ice chest or insulated container until serving time. Yield: 4 servings (servings size: 2 wedges). Cut each sandwich in half to serve.