1. In a bowl, stir vinegar, sugar, pepper and 1/2 tsp. salt until sugar dissolves. Stir in mustard seeds and cabbage.
2. Bring a pot of salted water to a boil. Add potatoes, return to a boil and cook until just tender, about 10 minutes.
3. Drain potatoes; let cool slightly. Fold into cabbage mixture with oil and chives. Serve warm or at room temperature.