Heat oven to 400° F. Whisk together the vinegar and sugar in a small bowl. Set aside.
Toss the potatoes, oil, rosemary, 1 teaspoon salt, and 1/4 teaspoon pepper in a 9-by-13-inch baking dish. Cover with foil and bake until just tender, 30 to 35 minutes. Add the butter and reserved vinegar mixture. Continue to roast, uncovered, basting occasionally with the pan juices, until the potatoes are golden brown, 25 to 35 minutes more.