Whisk together the soy sauce, vinegar, honey, garlic powder, ground ginger, green onion, and vegetable oil in a bowl; pour into a resealable plastic bag. Add the flank steak, coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator at least 4 hours.
Preheat an outdoor grill for medium-high heat, and lightly oil the grate.
Remove the flank steak from the marinade and shake off excess. Pour the remaining marinade into a small saucepan and set aside.
Cook the flank steak on the preheated grill until it begins to firm, and is reddish-pink and juicy in the center, about 5 minutes per side. An instant-read thermometer inserted into the center should read 130 degrees F (54 degrees C). Once cooked, cover the steak with several sheets of aluminum foil, and set aside to rest for 10 minutes before slicing thinly against the grain.
While the steak is resting, bring the marinade to a boil over medium-high heat. Pour the sauce over the sliced steak to serve.