1) Put potatoes into a large pot, cover with cold water by 2"to 3", and add a generous pinch of salt.
2) Bring to a boil over medium-high heat, reduce heat to medium and simmer until potatoes are tender about 15–20 minutes.
3) Meanwhile, cook bacon in a large frying pan over medium heat until crisp, 10–15 minutes.
4) Remove bacon from pan and drain on paper towels, keeping pan with rendered fat aside.
5) Crumble bacon into large pieces.
6) Drain potatoes and put into a large warm serving bowl and add bacon and onions.
7) Return pan with bacon fat to medium-high heat and heat until hot.
8) Carefully add vinegar and salt and pepper to taste.
9) Pour hot dressing over potato mixture and toss well.
10) Adjust seasonings. Garnish with parsley. Serve warm.