In a medium bowl, combine cilantro, vinegar, lime juice, oil, salt, pepper, and sugar; whisk until salt and sugar are dissolved. Add flank steak pieces, turning to coat well. Cover and marinate in refrigerator for 4 to 24 hours, turning meat occasionally.
Remove steak from marinade, discarding marinade. For a charcoal grill, grill steak on the rack of an uncovered grill directly over medium coals for 17 to 21 minutes for medium doneness (160 degrees F), turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place steak on a grill rack over heat. Cover and grill as above.)
Transfer steak to a cutting board and cover loosely with foil; let stand for 10 minutes. Cut steak against the grain into thin strips; cut strips with the grain into 2-inch pieces. Transfer steak pieces to a bowl and toss with any juices left on the cutting board.