Combine lemongrass and remaining ingredients except cooking spray in a food processor; process until smooth (about 1 minute). Combine beef and lemongrass mixture in a large zip-top plastic bag. Seal and marinate in refrigerator 2 to 4 hours.

Prepare grill.

Remove beef from bag; discard marinade. Place beef on grill rack coated with cooking spray; grill 1 minute on each side or until desired degree of doneness.

Bring a large pot of water to a rolling boil. Add the rice vermicelli and stir gently to loosen them. Cook until the noodles are white and soft but still slightly resilient, about 4 minutes. Drain and rinse under cold running water. Gently fluff the noodles and set them aside for at least 30 minutes. The noodles should be dry and sticky before serving.

Gently toss together the lettuce, bean sprouts, cucumbers, perilla, and basil leaves. Divide the salad mixture among 4 bowls. Top each with one-quarter of the rice noodles. The bowls are now ready for the topping.