Soak 8 bamboo skewers in cold water for 30 minutes.
Whisk together lemongrass, fish sauce, sugar, garlic, and oil. Slice beef very thinly against the grain. Add to marinade, and toss. Let stand at room temperature for 20 minutes.
Preheat a grill pan over medium-high heat. Thread beef onto skewers. Grill until browned, about 1 minute per side. Transfer to a platter, tent with foil, and let rest 5 minutes. Sprinkle with peanuts. Serve with the remaining ingredients on the side, bundling the meat, cucumber, and herbs inside the lettuce leaves if desired.