In a mixing bowl, combine vegetable oil, soy sauce, fish sauce, sesame oil, brown sugar or honey, minced garlic, and lemongrass pieces together to create a flavorful marinade for your chicken cubes.
Cook dried vermicelli noodles according to package instructions until al dente; rinse under cold water and drain well before setting aside.
Heat a pan over medium-high heat and sauté marinated chicken along with sliced onion until cooked through and golden brown; remove from heat once done.
In serving bowls, layer cooked vermicelli noodles at the bottom followed by sautéed chicken pieces, julienned cucumber, carrots, fresh cilantro, and roasted peanuts on top.
Whisk together fish sauce, rice vinegar, water, sesame oil, granulated sugar, black or white pepper, and red chili in a small bowl until well combined; drizzle over your assembled bowls.
Add lime wedges on each bowl for squeezing over before serving; mix everything together just before enjoying this nutritious meal!