cook potatoes and eggs in a large pot of boiling salted water until potatoes are fork tender, 12 to 15 minutes.
Drain and rinse eggs under cold water. Set eggs aside. Transfer potatoes to a large bowl and toss with vinegar. Cool slightly, about 15 minutes. Peel, then quarter or slice eggs, and add to potatoes.
Whisk together mayonnaise and Dijon. Add mayonnaise mixture to potato mixture along with celery and scallions. Mix gently. Season salad with salt and pepper and chill until ready to serve. Makes eight servings.