For the spicy chicken thighs, whisk the fish sauce, pepper, garlic, chillies and sugar together in a bowl.
In a separate bowl, pour half the marinade over the chicken thighs. Cover with cling film and refrigerate for 20 minutes. Reserve the rest of the marinade.
For cucumber and cashew salad, make a dressing by adding the lime juice and sugar to the reserved marinade, stirring until the sugar has dissolved.
In a heat-proof bowl, cover the vermicelli with boiling water and leave for a minute or so until the noodles have softened.
Drain and rinse the noodles under cold water. Mix the drained noodles in a large bowl with the cucumber, mint, spring onions, cashews and the dressing until well combined.
Remove the chicken thighs from the marinade, shaking off any excess. Fry the chicken thighs in the oil in a large frying pan over medium-high heat for 3-4 minutes on each side, or until cooked through. (You may have to do this in batches.)
To serve, divide the cucumber and cashew salad among four plates, slice the chicken and place alongside.