Cut off top 1/4 of each potato lengthwise; discard tops. Using fork, gently mash pulp inside each potato. Sprinkle with salt and pepper. Melt remaining 2 tablespoons butter in small saucepan over low heat. Whisk in remaining 1 1/2 tablespoons vinegar. Drizzle about 1 teaspoon butter-vinegar mixture into each potato; mix with fork to blend.
04Mound cranberry mixture in sweet potatoes. (Can be prepared 4 hours ahead. Cover loosely with plastic wrap; let stand at room temperature. Rewarm sweet potatoes uncovered in 350°F oven until heated through, about 20 minutes.)