Combine the fish stock, lemongrass, garlic and chilli in a saucepan. Bring to a simmer. Partly cover and cook for about 5 minutes.
Stir in the mushrooms and prawns. Simmer, partly covered, for a further 3 minutes or until the mushrooms are soft and the prawns have turned pink.
In a small bowl, combine the vinegar and soy sauce with the cornflour until smooth. Stir this mixture into the simmering soup, then bring back to the boil. Remove from the heat, stir in the spring onions and serve.