IN a large saucepan, combine potatoes, salt, and enough cold water to cover the potatoes. Bring to boiling; reduce heat. Simmer, covered, about 10 minutes or just until potatoes are tender. Drain well; cool slightly.
MEANWHILE, for the dressing, in a large bowl stir together the vinaigrette and ranch dressing. Add the onion, bell pepper, celery, scallions, and chives. Coarsely chop two of the hard-boiled eggs and add to the salad along with the potatoes; stir gently to coat. Cover and chill at least 4 hours or up to 8 hours.
TRANSFER the potato salad to a serving bowl. Halve the remaining two eggs and place on top of the salad, yolk-side up. Sprinkle with black pepper.
Instructions for Hard-Cooked Eggs: Place eggs in a medium saucepan and cover with water. Bring to a rolling boil; reduce heat and simmer for 1 minute. Remove from heat; cover and let stand for 12 minutes. Drain and cover with cold water. Let stand until completely cooled. Crack and peel the eggs.