Mix the lemongrass, fish sauce, brown sugar, neutral oil and garlic, then marinate the sirloin for 20 minutes.
Stir the carrot, cucumber, rice vinegar, granulated sugar and salt together and let the vegetables pickle for 20 minutes.
Heat a griddle over high heat and cook the steak in two uncrowded batches for 2 to 3 minutes each until browned and at least 145°F.
Drain the pickled vegetables thoroughly and squeeze them gently to remove excess brine.
Spread 1 tablespoon mayonnaise inside each baguette section and divide the hot steak among them.
Top each with 1/2 cup pickled vegetables and close the baguettes just before serving.