Combine the fish sauce, soy sauce, brown sugar/honey, minced garlic, onion, vegetable oil, black pepper, and lemongrass in a bowl. Add the pork slices and mix thoroughly. Marinate for a minimum of 30 minutes, or preferably overnight for the best flavor.

Grill the marinated pork or pan-fry it over medium-high heat. Cook until the slices are golden brown and slightly crispy around the edges.

Cook the rice vermicelli noodles according to the package directions. Drain the cooked noodles, then rinse them immediately with cold water and drain again completely.

In a small bowl, whisk together the fish sauce, lime juice/rice vinegar, sugar, water, minced garlic, and sliced chili (if using). Continue stirring until the sugar has fully dissolved. Taste and adjust the balance of sweet, sour, and savory to your preference.

In each serving bowl, create a base layer of the cooked rice noodles. Layer with the shredded lettuce, cucumber, carrots, and fresh herbs. Top with the grilled pork slices and a sprinkle of crushed peanuts. Drizzle generously with the Nước chấm or serve the sauce on the side.