Boil potatoes: In a large saucepan cover potatoes with salted cold water by 1-inch and simmer, covered, until just tender, about 15 to 30 minutes, depending on size of potatoes. In a colander drain potatoes and cool to warm.
With a sharp knife peel warm potatoes and cut into 1/3-inch-thick slices. In a bowl, immediately toss potatoes with vinegar.
In a separate bowl, mash yolks, (reserving whites) and stir in mayonnaise, mustard, and onion. Chop whites and celery and gently toss together with dill pickle and potatoes, mayonnaise mixture, and salt and pepper, to taste. Garnish with chopped parsley or scallions. Serve potato salad chilled or at room temperature.