Cook potatoes in pot of boiling water until just tender. Drain. Transfer to large bowl. Add 2 tablespoons vinegar to hot potatoes and stir gently. Combine remaining 1 1/2 tablespoons vinegar and mustard in small bowl. Gradually mix in oil. Add chopped dill. Mix into potatoes. Season with salt and pepper. (Can be made 6 hours ahead. Cover; chill. Bring to room temperature before serving.) Mix in cucumbers. Garnish with dill sprigs.
