Preheat your sous vide water bath to 65 degrees C.

Place chicken thighs in a vacuum bag or a resealable freezer food storage bag along with the brown sugar, fish sauce, and chiles. Seal the bag and lower the bag into the water bath. Cook, fully submerged (weight bag if necessary), for 1 ½ to 2 hours. Remove the bag from the water bath and remove the chicken from the bag; set aside. Pour the cooking liquid into a separate bowl.

Heat a large nonstick skillet over medium high heat and add 1 tablespoon of the oil. When hot, add the chicken thighs and cook until nicely caramelized in spots, 2 to 3 minutes. Transfer chicken to a platter. Add the bell pepper and onion slices to the hot pan and cook until crisp-tender, 1 to 2 minutes. Transfer them to the platter with the chicken.

Add the remaining tablespoon of oil to the pan and add the lemongrass, garlic, and ginger. Cook until fragrant and softened, about 2 minutes. Pour the reserved cooking juices into the pan and allow to cook until reduced to a syrupy consistency, 3 to 4 minutes. Add the vinegar and lime juice to the pan and cook until slightly thickened, 1 to 2 minutes longer. Return the chicken and veggies to the pan, add the basil leaves and toss to combine. Remove from the heat and serve with rice or noodles.