Place the potatoes and salt in a medium pot and add enough cold water to cover by 2 inches. Bring to a boil. Reduce heat and simmer until the potatoes are fork-tender, about 15 minutes. Drain and transfer to a serving dish. Whisk together the vinaigrette and mustard in a small bowl. Pour half the mixture over the hot potatoes, toss, and let stand for 1 hour. Just before serving, drizzle the remaining vinaigrette mixture over the potatoes, add the tarragon and pepper, and toss.
Tip: Drizzling the vinaigrette over the potatoes while they're hot helps them absorb the tasty liquid more readily.