Cook vermicelli noodles according to package directions. Set aside.
To prepare beef: In medium bowl, whisk garlic, ginger, fish sauce, brown sugar, lemongrass paste and lime juice until blended. Add beef and mix until well blended. Marinate 15 minutes.
In large skillet over medium-high heat, heat vegetable oil. Add beef and cook, breaking up using wooden spoon and stirring occasionally, until beef is dark and caramelized, 10-12 minutes.
To make dressing: In small bowl, combine soy sauce, lime juice, lemongrass paste, brown sugar and sriracha, if desired. Whisk until sugar dissolves. Slowly stream in vegetable oil, whisking, until dressing is thickened.
To prepare salads: Divide noodles between four shallow bowls. Arrange caramelized beef, lettuce, carrots, cucumber, mint and cilantro in bowls then drizzle with dressing and sprinkle with peanuts. Serve with lime wedges for squeezing on top.
