Place potatoes in a large pot; cover with cold water by 1 inch. Salt water, cover and bring to a boil. Reduce to a simmer and cook until potatoes are tender, about 10 minutes; drain.
In a small bowl, whisk together mayonnaise, sour cream, 1 Tbsp. vinegar and 1/4 tsp. salt.
Transfer potatoes to a bowl and sprinkle with 2 Tbsp. vinegar. Toss lightly. Add dressing, chives and pepper; toss. Cover and refrigerate at least 1 hour and up to 24 hours.
