In a bowl, combine fish sauce, minced garlic, brown sugar, and vegetable oil. Add chicken thighs and marinate for at least 30 minutes, or up to 4 hours in the refrigerator.
Heat a grill pan or skillet over medium-high heat. Cook marinated chicken for 6-8 minutes per side until internal temperature reaches 165°F. Let rest for 5 minutes, then slice thinly.
Toss julienned daikon and carrots with 1 teaspoon salt. Let sit for 10 minutes, then drain and rinse.
In a small saucepan, combine rice vinegar, water, sugar, and remaining salt. Bring to a boil, stirring until sugar dissolves.
Pour hot pickling liquid over vegetables. Let cool to room temperature, about 20 minutes.
Mix mayonnaise with Sriracha sauce in a small bowl.
Cut baguettes in half lengthwise and lightly toast if desired. Spread Sriracha mayo on both sides of bread.
Storage tip: Pickled vegetables keep in the refrigerator for up to 2 weeks. Assemble sandwiches just before serving for best texture. For meal prep, store all components separately.