Preheat a grill or grill pan over medium-high heat. Rub the flank steak with vegetable oil, salt, and black pepper.
Grill the steak for about 4-5 minutes on each side for medium-rare, or until it reaches your desired doneness. Remove from the grill and let it rest for 5 minutes.
While the steak is resting, prepare the salad. In a large bowl, combine cherry tomatoes, cucumber, red bell pepper, red onion, cilantro, and mint.
In a small bowl, whisk together lime juice, fish sauce, sugar, and red pepper flakes until the sugar is dissolved.
Slice the rested steak thinly against the grain and add it to the salad bowl. Pour the dressing over the salad and toss gently to combine.
Serve immediately, garnished with extra herbs if desired.