Place potatoes in a large pot and cover with about 2 inches of COLD water and 2 tablespoons of salt. Bring to a boil over high heat, then lower the heat to a simmer and cook for about 5-10 minutes or until the potatoes are just fork tender.
While the potatoes are cooking, cook the sliced bacon in a skillet over medium heat until crispy and most of the fat has rendered out. Use a slotted spoon to remove the bacon from the pan, placing on a layer of paper towels to drain. Do not discard the bacon grease.
Add the onion to the pan with the bacon fat and cook over medium heat until the onions are soft, 3-5 minutes. Once onions are soft, remove from the heat and add the mustard and 4 tablespoons of the vinegar to the pan. Season with a pinch of salt and pepper to taste and whisk to combine.
When the potatoes have finished cooking, drain them and spread them onto a large baking sheet. While they are still hot, drizzle them with 1-2 tablespoons of the remaining vinegar and sprinkle with a couple of pinches of salt. Allow to cool for about 10 minutes.
Place the potatoes in a large bowl and add the dressing from the pan, tossing to combine. Fold in the cooked bacon and chives and taste for seasoning, adding more salt, pepper, and/or vinegar as needed.
Serve warm or at room temperature.
Notes
Seasoning the potatoes with vinegar and salt while they are still hot is the key to this recipe. After you have seasoned the potatoes, let them rest for a couple of minutes, then taste and add more vinegar or salt if desired.