Even better with homemade stock, although canned broth will do!
Wash the leeks and cut them down to 2 inches below where the green leaves separate from the whole stem. Slice the lower stems and bulbs lengthwise in half, then crosswise about ¼-inch thick. Rinse thoroughly and drain. Melt butter in a heavy soup pot and saute the leeks gently, uncovered, for about 15 minutes. Don’t let them brown. Add flour and stir well. Cook at very low heat for 10 minutes more, stirring occasionally.
Meanwhile, in a large saucepan combine chicken stock, salt, and pepper. Bring to a boil, turn down the heat, and ladle 1 cup of the stock into the leek-and-flour mixture in the soup pot. Mix well. Add 2 cups milk to the leek mixture. Cook the mixture gently, stirring often, for another 10 minutes. Turn off the heat. Bring the remaining chicken stock to a boil, add the vermicelli, and cook for about 5 minutes, until the vermicelli is soft. Add the stock and vermicelli to the soup pot, and heat through.