Cut off dark green tops of leeks and trim root ends. Halve each leek lengthwise to within 2 inches of root end. Rinse well under cold running water to wash away sand. Cover leeks with cold water in a 12-inch heavy skillet. Add salt and simmer leeks, uncovered, until tender, 15 to 20 minutes. Transfer leeks to a bowl of ice and cold water to stop cooking, then drain and pat dry with paper towels. While leeks simmer, finely chop pancetta, then cook in 1 tablespoon oil over moderately low heat, stirring occasionally, until beginning to brown, about 5 minutes. Whisk together vinegar, mustard, remaining 1/4 cup oil, and salt and pepper to taste in a bowl until emulsified. Add leeks and gently toss to coat. Serve topped with pancetta and parsley.