Soak vermicelli noodles in hot boiling water until soft and translucent, about 5 minutes. Drain and rinse the noodles under cold running water. Set aside.

In a large bowl, whisk together the lime juice, fish sauce and sugar. Add the noodles and toss until the noodles are fully coated with the sauce.

Add onions, chilis, tomatoes, cilantro and peanuts; toss to combine.

Serve immediately or chill first in the fridge for half an hour before serving.