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