Firm and slice the pork. Place the pork in the freezer for 30–45 minutes, or until firm but not frozen solid. Slice it across the grain into pieces about 3–5 mm thick.
Make the marinade. In a large bowl, combine the minced lemongrass, garlic, fish sauce, sugar, chilli, salt, pepper, MSG (if using) and vegetable oil. Stir until the sugar begins to dissolve.
Marinate the pork. Add the sliced pork and mix thoroughly.. Cover and refrigerate for at least 30 minutes, preferably 2–4 hours. Remove it from the fridge about 15 minutes before cooking.
Cook the vermicelli. Prepare the noodles according to the packet directions. Rinse thoroughly under cold water and drain very well.
Prepare the vegetables and herbs. Shred the lettuce, cut the cucumber and pick the herb leaves. Arrange everything on platters for people to help themselves.
Preheat the grill. Heat a barbecue, grill pan or heavy frying pan over high heat. Lightly oil the cooking surface.
Cook the pork. Arrange the pork in a single layer and cook in batches for about 2–3 minutes per side, or until cooked through with caramelised, lightly charred edges. Do not crowd the pan or grill. Transfer to a plate and rest for 3–5 minutes.
Build the bowls. Add the warm grilled pork to the noodles, lettuce, cucumber and herbs. Make sure everyone gets some of the charred pieces—assuming they have survived the trip from grill to table.
Dress and eat. Spoon over Nuoc Cham, toss everything together and add fresh chilli if wanted. Eat immediately.
