Bring a large pot of salted water to a boil. Add sweet potatoes, and return to a boil. Reduce heat, and simmer until tender, about 5 minutes. Drain.
Whisk vinegar, orange zest and juice, 1 scallion, and 1 teaspoon salt in a small bowl. Add oil in a slow, steady stream, whisking until emulsified. Season with pepper. Toss dressing with sweet potatoes in a large bowl, and garnish with remaining 1/2 scallion. Refrigerate until chilled, up to 1 day.