Whisk the fish sauce, pepper, garlic, chillies, and sugar in a bowl.
Pour half the marinade over the chicken thighs, cover, and refrigerate for 20 minutes.
Stir lime juice and sugar into the remaining marinade to create the salad dressing.
Cover vermicelli with boiling water in a bowl for one minute until softened.
Drain noodles, rinse with cold water, and toss with cucumber, mint, onions, cashews, and dressing.
Fry the chicken thighs in vegetable oil for 4 minutes on each side until cooked through.
Slice the chicken and serve alongside the noodle salad.