1.Place vermicelli in large heatproof bowl, cover with boiling water, stand until just tender, drain. Cut into random lengths.
2.Make dressing. Place ingredients in screw-top jar, shake well.
3.Shell and devein prawns, halve prawns lengthways.
4.Place vermicelli and prawns in large serving bowl with onion, capsicums, cucumber, herbs and dressing, toss gently to combine. Top with shallots, serve immediately.