Cook potatoes in large pot of boiling salted water until just tender, about 25 minutes. Drain; cool. Cut potatoes into quarters. Transfer to large bowl; add vinegar and toss to coat. Mix in celery and peas. Whisk mayonnaise, 5 tablespoons chives, mustard, garlic, and cayenne pepper in small bowl to blend. Add to potato mixture and toss. Season generously with salt and pepper. Cover and chill at least 1 hour to allow flavors to blend. (Can be made 1 day ahead. Keep refrigerated.) Sprinkle with remaining 1 tablespoon chives and serve.