To a large pot, add the potatoes, 1 cup of vinegar, and the tablespoon of salt. Add water to the pot, until it covers the potatoes by 2 inches. Bring the pot to a rolling boil, reduce heat to medium low, and then simmer the potatoes for 20 minutes, or until fork tender.
Transfer the potatoes to a strainer to drain, and let them rest there for a minute or two while the steam dries them slightly.
Heat the butter in a large cast iron skillet, over medium high heat, until melted and hot. Stir in the potatoes, season with salt & pepper, and fry until the potatoes are golden brown & crispy.
Transfer the fried potatoes to a serving dish, and drizzle with the remaining vinegar. Sprinkle with the flaked salt, and then garnish with the chives- if using.