In a pot with a steamer insert, steam the potatoes until tender, about 15 minutes. (The potatoes can also be cooked in boiling salted water for the same amount of time.) Remove from the heat and let cool to room temperature.
In a pot of boiling water, blanch the leeks for 30 seconds, or until tender. Rinse in cold water and drain. Set aside.
Cut the roasted peppers in 1/2 crosswise and then cut them into 1/2-inch slices.
Cut each potato into 8 pieces. In a medium bowl, mix the potatoes, leeks, peppers, thyme, and tarragon. In a separate bowl, stir together the Champagne vinaigrette with the mustard. Drizzle the vinaigrette on the potato salad and toss gently until the vinaigrette coats the salad evenly. Season with the salt and pepper and toss again.
In a small bowl, whisk the vinegar and salt together until the salt dissolves. Slowly whisk in the oil until the dressing is well blended. Season with pepper, to taste.
Recipe courtesy Debra Ponzek and Geralyn Delaney Graham, The Summer House Cookbook, Potter, 2003