While potatoes are cooking, add vinegar, celery, onions and a pinch of salt to a large bowl. Mix and set aside. Add warm potatoes to bowl and mix gently. Leave to cool. When cooled, add mustard, garlic powder, relish, mayonnaise, 1/2 tsp each coarse salt and ground black pepper. Mix well. Add chopped eggs and mix. Taste and adjust salt if necessary. If desired, mix in thinly sliced scallions and sprinkle paprika on top. Cover and refrigerate for at least one hour. This creamy red potato salad tastes even better the next day.