Combine the soy sauce, vinegar, brown sugar, Sriracha and garlic in a large bowl. Add the beef and onion and toss to coat; cover and refrigerate 1 to 4 hours.
Preheat a grill to medium high and brush the grates with vegetable oil. Thread the beef, onion and scallions onto eight 10-inch skewers and brush with more oil. Grill the kebabs, turning occasionally, until marked, about 7 minutes for medium rare.