Mix together the prawns, cucumber, herbs, vermicelli and lettuce in a large bowl.
Soak 1 rice sheet in a bowl of warm water for 20 seconds, then drain on kitchen paper. Fill with the prawn mixture, leaving about 2.5 cm (1 inch) at the top and the bottom of the sheet. Fold over the top and bottom edges and roll up. Repeat with the remaining rice sheets and filling.
Whisk together all the dipping sauce ingredients in a serving dish and serve with the rolls and lime wedges.