Put the potatoes in a medium saucepan and cover with cold water. Bring to a boil over medium-high heat and cook until easily pierced with a fork, 6 to 7 minutes. Drain and cool completely, about 30 minutes.
Meanwhile, cook the bacon in a medium skillet over medium-high heat until golden brown and crispy, about 7 minutes. Transfer to a paper towel-lined plate to drain. Transfer 2 tablespoons bacon to a small bowl and reserve for topping.
Whisk together the sour cream, mayonnaise and vinegar in a large bowl until smooth. Fold in the cooled potatoes, Cheddar, scallion whites, remaining bacon, 1 1/2 teaspoons salt and a few grinds of black pepper. Transfer to a serving dish and sprinkle with the scallion greens and reserved 2 tablespoons bacon. Chill for about 30 minutes before serving.
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