Cook potatoes by boiling them in a large pot of water until fork tender, about 15 minutes. Make sure not to over cook them and give them a good stir halfway through. When they are finished, rinse them with cool water.
Combine the mayonnaise, yogurt, dill, vinegar, salt, and pepper.
Add the dressing to the cooked potatoes, egg, celery, and scallions. Chill for at least two hours before serving.