In a medium bowl, combine the pork with the shallot, lemongrass, 3 minced garlic cloves, 3 tablespoons fish sauce, 2 tablespoons brown sugar, 1 teaspoon oil, and black pepper. Toss well. Cover and refrigerate for at least 20 minutes while preparing the remaining components.
Make the dressing by whisking the warm water, lime juice, rice vinegar, remaining 3 tablespoons fish sauce, remaining 2 tablespoons brown sugar, remaining minced garlic clove, and optional chile until the sugar dissolves. Set aside.
Cook the rice vermicelli according to the package directions until tender. Drain, rinse thoroughly under cold water, and drain again. Toss the noodles with the remaining 1 teaspoon oil to limit sticking.
Heat an outdoor grill or grill pan over medium-high heat. Lightly oil the grates or pan. Lift the pork from the marinade, letting excess drip off, and cook in a single layer in batches until browned and cooked through, about 3 to 4 minutes per side. Cook pork to an internal temperature of 145°F (63°C), then let it rest for 3 minutes.
Divide the lettuce, noodles, cucumber, carrot, herbs, and grilled pork among four bowls. Sprinkle with peanuts. Serve the dressing on the side or spoon it over each bowl just before eating.