Preheat grill to medium-high. Whisk vinegar, sugar, and 1/2 teaspoon kosher salt together until sugar is dissolved. pour marinade into a resealable plastic bag, add steak, seal, and marinate 15 minutes, turning the bag occasionally.
Process parsley, cilantro, oil, lime juice, salt, and a pinch of red pepper flakes in a food processor, blending until herbs are minced; set aside.
Remove steak from bag (reserve the marinade) and grill 6 minutes on one side. Flip, baste with reserved marinade, and grill 6 more minutes for medium-rare (or longer for desired doneness).
When the steak is nearly done, turn, baste the other side and cook an additional minute.
Remove from grill and let rest 5 minutes, then slice steak diagonally across the grain into thin slices.
Serve with parsley-cilantro sauce. Serve with fresh green beans