For the nuoc cham, place all of the nuoc cham ingredients into a food processor and blend until smooth.
For the crab salad, soak the rice noodles in very hot water for five minutes, or until soft (or cook according to packet instructions - do not overcook). Drain and set aside to cool.
Place all of the remaining salad ingredients into a bowl and mix well, seasoning, to taste, with salt and freshly ground black pepper.
To serve, drizzle a little of the nuoc cham dressing into the noodles and mix well.
Fold half of the crab salad mixture into the noodles, then using a large fork, wrap up a forkful of noodles and gently place into a small bowl to create a 'nest' of noodles. Repeat with three more bowls.
Spoon some of the nuoc cham dressing over each noodle nest.
Top with a spoonful of crab salad mixture, then spoon over a little more dressing.