Toss carrot and daikon with rice vinegar, granulated sugar, and 1/2 teaspoon salt. Let stand 30 minutes.
Mix pork with lemongrass, fish sauce, brown sugar, oil, and remaining salt. Let stand 15 minutes.
Heat a wide skillet over medium-high heat. Cook pork in 2 batches, 2 to 3 minutes per side, until browned and 145°F.
Rest pork 3 minutes while splitting baguettes, keeping hinges intact.
Spread baguettes with mayonnaise. Add pork, drained carrot-daikon pickle, cucumber, cilantro, and jalapeño.
Close lightly to keep crust intact; serve before bread softens.