Cook potatoes in boiling, salted water until very tender, 15 to 20 minutes. Drain; return to saucepan and mash coarsely. Cook bacon in a large heavy skillet over medium heat until browned; drain and crumble. Pour off drippings from skillet, leaving clinging particles in skillet. Add potatoes, salad dressing, buttermilk and vinegar to skillet; stir until well blended. Cook, stirring constantly, until hot. Add salt and pepper to taste.