To prepare dressing, combine the first 8 ingredients in a small saucepan; cook over medium heat 5 minutes or just until sugar dissolves. Remove from heat; cool.
To prepare salad, place vermicelli in a large bowl. Cover with boiling water. Let stand 20 minutes or until tender. Drain and rinse under cold water; drain.
Cut tenderloin in half lengthwise. Cut each piece in half crosswise. Place each pork piece between 2 sheets of plastic wrap; pound to an even thickness using a meat mallet or small heavy skillet. Combine 2 teaspoons fish sauce and 1 teaspoon Sriracha; drizzle over pork. Sprinkle evenly with garlic powder, salt, and pepper. Pat brown sugar onto pork.
Place pork on a grill rack coated with cooking spray. Grill 12 minutes or until slightly pink in center, turning pieces occasionally to prevent burning. Place pork on a cutting board; let stand 5 minutes. Cut across grain into very thin slices.
Combine vermicelli, lettuce, and next 6 ingredients (through mint) in a large bowl. Pour dressing over salad; toss well. Top with pork and nuts. Serve with lime wedges, if desired.
Beer note: Try a Belgian white beer, a light-bodied wheat beer that both refreshes and invigorates. The widely available Hoegaarden White ($8 for a six-pack) has a sweet-tart balance and includes subtle additions of orange peel and coriander--ingredients that bring their own exotic spice to this dish's mélange of Asian flavors. --Jeffery Lindenmuth