Cook whole red potatoes in boiling water until you can easily poke through them with a paring knife (but not to the point that they are falling apart). Large-ish potatoes (so that it took 7 to make the 2 pounds) took about 25 minutes. Adjust up or down depending on the size of your potatoes.
Drain the potatoes, rinse in cold water, drain again and let cool a bit. Cut into 1/2-3/4” inch chunks. I like skin on but you can take the skin off before cutting if you don’t like the skin. Toss the still warm potatoes with the vinegar, salt and pepper.
In a small bowl blend the mustard, sour cream and mayonnaise until smooth.
Add the celery, pickles and onions and/or scallions to the potatoes. Stir in the dressing and mix thoroughly. Sprinkle the parsley on top and mix in a bit. Chill an hour or overnight, covered. Sprinkle with paprika just before serving.