Stir together the fish sauce, soy sauce, sugar, and oil until the sugar is completely dissolved. Add the lemongrass, shallot, garlic and pork and mix to coat the meat evenly. Allow to marinate at room temperature for 20 minutes or refrigerate for at least 1 hour or up to overnight.
Slide 2 to 4 slices of pork onto each skewer so the meat is flat with the skewer going through the slices several times. Grill over a barbecue (make sure that the flames have subsided and the coals are red with white ashes). Alternatively, heat a well-oiled grill pan or non-stick skillet over high heat and, working in batches, cook the skewers until the edges crisp, about 1 minute per side. Remove the skewers from the grilled pork.
Divide the grilled pork among the bowls of rice vermicelli. Sprinkle peanuts and drizzle nuoc cham over each serving. Serve immediately.
Fish Dipping Sauce: Nuoc Cham
Whisk together the sugar, water, fish sauce, and lime juice in a bowl until the sugar is completely dissolved. Add the garlic, chile, and shallot, and let stand for 30 minutes before serving.