Toss the carrot and daikon with the rice vinegar, granulated sugar and 1/2 teaspoon salt, then let the pickles stand for 30 minutes.
Mix the pork with the lemongrass, fish sauce, brown sugar, oil and remaining salt and let it stand for 15 minutes.
Heat a wide skillet over medium-high heat and cook the pork in 2 batches for 2 to 3 minutes per side until browned and it reaches 145°F.
Rest the pork for 3 minutes while you split the baguettes without cutting through the hinge.
Spread the baguettes with mayonnaise, then add pork, drained carrot-daikon pickle, cucumber spears, cilantro and jalapeño.
Close each banh mi lightly so the crust stays intact and serve before the bread softens.