Boil the diced potatoes in salted water until tender, about 15-20 minutes.

Drain and let cool.

In a mixing bowl, combine the cooled potatoes, celery, and onion.

In a separate bowl, mix together the mayonnaise, vinegar, mustard, salt, and pepper.

Pour the dressing over the potato mixture and stir gently to combine.

Chill in the refrigerator for at least 30 minutes before serving.