First make the sauce. Heat the vinegar and sugar in a small saucepan until the sugar has dissolved and the mixture is becoming syrupy . Stir in the soy sauce and leave to cool.
Add the chillies, coriander leaves and lime juice then set aside.
Put the prawns, garlic, spring onion, lemon grass, coriander, chillies, flour and a little salt into a food processor and blitz to a rough paste. Set aside in the fridge for 30 minutes.
Press spoonfuls of prawn paste into patties or flat cakes- you should get 8.
Heat enough groundnut oil in a shallow pan to cover the base. Put in the prawn patties and let them colour before gently turning.
When they are crisp and golden on the outside but still moist in the middle lift out and lay on the bread rolls, spooning some sauce over the top.