Cut the potatoes lengthwise into fourths and in a steamer set over boiling water steam them, covered, for 7 to 10 minutes, or until they are just tender. In a bowl whisk together the mustard, the vinegar, the vermouth, and salt to taste, add the oil in a stream, whisking, and whisk the dressing until it is emulsified. Add the potatoes while they are still warm to the dressing and toss them gently with the dressing, the dill, and pepper to taste until they are coated well. Let the potato mixture stand, tossing it occasionally, for 30 minutes and serve it at room temperature. The potato mixture may be made 1 day in advance and kept covered and chilled. Let the potato mixture return to room temperature before serving.