In a small bowl, whisk together vinegar and 1/2 teaspoon salt until salt has dissolved; add mustard, capers, and oil and whisk to blend. Season with salt; set vinaigrette aside.
Bring 1 quart of water to a boil in the bottom of a saucepan fitted with a steamer basket. Place leeks in steamer basket, cover, and steam until cooked through, about 10 minutes.
Drain leeks and transfer to a serving platter. Immediately cover with vinaigrette. Sprinkle with chives or parsley and serve immediately.