In a small saucepan, combine rice vinegar, water, sugar, and salt. Heat over medium until sugar dissolves (about 2-3 minutes). Remove from heat and let cool for 5 minutes. Place the julienned carrots, daikon, and cucumber in a bowl, pour the pickling liquid over them, and set aside for at least 30 minutes (or refrigerate overnight for best results).

In a bowl, combine soy sauce, fish sauce, honey, minced garlic, grated ginger, lemongrass paste, and vegetable oil. Add the chicken thighs, ensuring they're well coated. Cover and refrigerate for at least 30 minutes, or up to 4 hours for deeper flavor.

In a small bowl, whisk together mayonnaise, sriracha, lime juice, and sesame oil until smooth. Adjust the sriracha to your preferred level of spiciness. Refrigerate until ready to use.

Heat a grill pan or skillet over medium-high heat. Remove chicken from marinade and cook for 4-5 minutes per side, or until fully cooked (internal temperature of 165°F/74°C). Let rest for 5 minutes, then slice into thin strips.

Preheat oven to 350°F (175°C). Slice baguettes lengthwise, leaving one side connected. Toast in the oven for about 3-5 minutes until just crispy on the outside but still soft inside.

Spread a generous layer of sriracha mayo on both sides of each baguette. Layer in the sliced chicken, drained pickled vegetables, jalapeño slices, cilantro leaves, and mint leaves (if using). Gently press the sandwich together.

Cut the sandwich in half diagonally for easier eating and serve immediately while the bread is still crispy and the fillings are fresh.