Mix the plum sauce with the rice vinegar and soy sauce in a small bowl and set aside.
Pour plenty of freshly boiled water into a medium saucepan and bring back to the boil. Add the noodles and cook for 3 minutes. Drain through a colander.
Heat the oil in a large frying pan or wok for 1 minute. Add the spring onions and chilli and fry for 1 minute, stirring with a wooden spoon. Add the pak choi or cabbage and the sweetcorn and fry for a further 2–3 minutes until the vegetables are softened.
Tip the noodles, prawns and sauce into the pan and cook, stirring gently, over a low heat until the ingredients are mixed together and hot. Serve immediately.