In a mixing bowl, combine the beef, lemon grass, spring onions, garlic, salt and pepper. Allow flavors to infuse for at least 15 minutes.
Meanwhile, pick the individual betel leaves and wash in cold water. Lay the leaves flat on a cloth to dry.
To form the rolls, lay a large betel leaf (or 2 smaller leaves), shiny side down, on a board with the stem of the leaf pointing towards you. Spoon approximately 1 tablespoon of the beef mixture onto the bottom edge of the leaf. Work it into a sausage shape, then roll the leaf from bottom to top and place the seam flat on your bench to stop the leaf from unrolling. Repeat this process until you have used all of the beef. The mixture should make about 20 rolls.
Cook the parcels, seam down first, on a grill or barbecue hotplate over medium heat, turning to color all over, for about 5 minutes, or until done.
Place the cooked parcels on a bed of vermicelli noodles, and then drizzle with a tablespoon of nuoc mam dipping fish sauce and garnish with fried red shallots, peanuts and chilli.
Combine the fish sauce, rice vinegar, sugar and 1/2 cup of water in a saucepan and place over medium heat. Stir well and cook until just before boiling point is reached, then remove from the heat and allow to cool. To serve, add the garlic and chilli, then stir in the lime juice. Store tightly sealed in a jar in the fridge, for up to 5 days. Yield: about 1 cup