In a bowl, whisk together sweet chili sauce, soy sauce, brown sugar, sriracha, rice vinegar, fish sauce, garlic, lemongrass paste, lime juice, and sesame oil to make the glaze.
Heat a skillet over medium-high heat and sear chicken tenderloins until cooked through and golden, about 5–6 minutes per side.
Pour glaze over the chicken and simmer until the sauce thickens and coats the chicken.
Meanwhile, combine vinegar, salt, sugar, red pepper flakes, and garlic in a saucepan to heat slightly, then pour over carrots and cucumber to pickle.
Let the pickled veggies sit for at least 10 minutes while the chicken rests.
Serve the Thai chicken hot with pickled veggies on the side or over jasmine rice for a full meal.
Garnish with chopped herbs, crushed peanuts, or lime wedges if desired.