Combine the lemongrass, chilli, garlic, fish sauce, sugar and vegetable oil in a large bowl, and stir until the sugar has dissolved. Add the beef, mix well and set aside in the fridge to marinate for 3 hours.
Cook the noodles according to the packet instructions, then drain and run under cold water until cool. Drain again and set aside.
Place all the salad plate ingredients and cooked noodles on a serving platter. Combine the Nuoc Mam and pickles in a bowl, then divide among dipping bowls. Pile the rice paper sheets on a plate and set a large bowl of water on the table.
Set up the hot plate in the centre of the table and heat over medium–high heat. Melt a little of the butter on the hot plate, add a few slices of marinated beef and cook for 2 to 3 minutes until just cooked through. Invite guests to help themselves, and continue to cook the remaining beef in small batches as you eat.
To eat, invite guests to rehydrate rice paper sheets in water, then fill with the noodles, salad ingredients and beef. Roll up the rice paper and dip into the Nuoc Mam and pickles.