Bring the potatoes, 1 tablespoon salt, and enough water to cover by 1 inch to a boil in a big pot over high heat.
Decrease heat to medium and simmer until the potatoes are just tender, about 10 minutes.
While the potatoes simmer, whisk the mayonnaise, buttermilk, 2 tablespoons of vinegar, the red peppers, cilantro, scallions, garlic, dill, 1 teaspoon salt and 2 teaspoons pepper in a big bowl.
Drain the potatoes, then spread them in an even layer on a rimmed baking sheet.
Whisk the mustard and remaining 2 tablespoons vinegar in a small bowl; drizzle the mustard mixture over the hot potatoes and toss until evenly coated; refrigerate the potatoes until cooled, about 30 minutes.
Transfer the cooled potatoes to a bowl with the mayonnaise mixture; toss to combine.
Cover and refrigerate until well chilled, about 30 minutes; serve.
This salad can be refrigerated in an airtight container for up to 2 days.