Start by soaking your vermicelli in a large bowl of boiling water for about 8-10 minutes until cooked. Drain, and then rinse through with cold water to prevent it from sticking together.
Next make your garnish by mixing the vinegar and sugar together until the sugar dissolves, then mixing in the chilli, cucumber and red onion. Put to one side.
Whilst your noodles are soaking, make your Laksa paste. Heat the oil in a medium-to-large pan, and add the chopped onions. Cook on medium for about 5 minutes until the onions soften and start to turn translucent. Add in the rest of the Laksa paste ingredients, stir and continue to cook for 6-7 minutes, stirring every now and then. Then carefully use a stick blender to blend the ingredients into a paste. You could also use a mini chopper to do this.
Add the coconut milk and stock to the paste and gently heat through. Then add in your fish, stir, and allow to cook for 2-4 minutes until the fish is starting to fall apart. Add in your prawns, stir, and cook for an extra one minute. The prawns should be starting to turn pink, and your fish should be cooked. Finally add in the beansprouts and cook for another minute.
Whilst your fish and beansprouts are cooking, divide your noodles between the bowls. Scoop the fish, prawns and beansprouts out of the soup, and spoon them over the noodles. Then gently pour or spoon the sauce over and around the noodles.
Garnish with a sprinkle of coriander and a teaspoon of the chilli/red onion/cucumber mix.