Chop the lemon grass into short lengths, split them in two and crush with a pestle and mortar.
Juice one of the limes and cut the other into 4 segments.
Dip the tomatoes in boiling water for 10 seconds then peel, seed and cut into 6 segments.
Place the ginger, halved garlic cloves, lime segments, tomatoes, lemongrass, chill and celery leaves in a saucepan.
Add the cold chicken stock; bring slowly to the boil and leave to simmer for 5 minutes.
Add the vermicelli and cook over a very low heat until tender. Combine with the bean sprouts, lime juice, and fish sauce and serve up immediately in little bowls with lime wedges.