1.Place vermicelli in a medium heatproof bowl; cover with boiling water. Stand until just tender, drain.
2.Meanwhile, to make the chilli dressing: Combine sugar and the water in a small saucepan; stir over low heat until sugar dissolves. Bring to the boil. Reduce heat; simmer, uncovered, for 5 minutes or until mixture thickens slightly. Remove from heat; stir in vinegar and chilli.
3.Heat oil in a wok over high heat; stir-fry chicken, in batches, for 2 minutes or until browned. Return all of the chicken to wok with lemongrass and garlic; cook, stirring, for 1 minute or until lemon grass is soft. Add fish sauce; stir-fry until hot.
4.Serve vermicelli with lettuce, carrot, cucumber and chicken mixture; sprinkle with nuts, radish and herbs. Drizzle over chilli dressing.
5.Serve with remaining dressing.
Recipe is not suitable to freeze or microwave.
You will need about half a medium iceberg lettuce to get the amount required for this recipe.