Toss potatoes on a baking sheet with a drizzle of oil, half the paprika, salt, and pepper. Roast until golden brown and crispy, 20-25 minutes. Meanwhile, in a small bowl, combine mayonnaise, mustard, 1 package sour cream (2 packages for 4 servings), scallion whites, remaining paprika, 1 tsp vinegar (2 tsp for 4), and a pinch of chili to taste. Season with salt and pepper. Set aside.