Soak vermicelli in a large bowl of cold water for 30 minutes. Drain the noodles, and set aside.
Make the pesto: In a blender or food processor combine the coriander, basil, garlic, lemon grass, chilli, fish sauce and 2 tablespoons of the peanuts. Process just until the herbs and peanuts are coarsely chopped. While the machine is running, add the oil in a thin stream. Then add the remaining peanuts and run the machine in short spurts until the peanuts are coarsely chopped.
Place soaked vermicelli into a large frying pan with 1/2 cup water over medium high heat. Stir until most of the water has been absorbed and the noodles are tender.
Add almost all of the pesto, and stir well, adding a few tablespoons of water if the pesto is clumping.
Taste the noodles and add more pesto, lime juice, fish sauce, salt or pepper if you like. Garnish the noodles with the remaining 2 tablespoons of peanuts, and serve right away.