In a large pot, boil eight cups of water. Add the clams and wait until they open.
When the clams open, scoop them out immediately and set aside.
Into the hot water, add the garlic, ginger and onion. Cover the pot and simmer for about ten minutes or until the vegetables have rendered their flavors into the broth.
Add the carrot slices and unsoaked vermicelli. Season with fish sauce. Cover and cook until the noodles swell, about ten minutes.
Turn up the heat. Return the clams to the pot and, as soon as the soup boils, turn off the heat. Add the wakame, cover the pot and leave for five minutes. Taste and season with more fish sauce, if needed.
To serve the clams vermicelli soup, place the noodles at the bottom of the bowl, top with the vegetables and clams then pour in the broth. Sprinkle with onion leaves and serve hot.