In a medium bowl, whisk together the white vinegar, soy sauce, BBQ sauce, Worcestershire sauce, sesame oil, garlic salt, and black pepper to make the marinade.
Place the cut steak in a large bowl or resealable plastic bag and pour the marinade over the meat, stirring or massaging to coat evenly.
Cover the bowl or seal the bag and refrigerate for at least 8 hours, stirring or turning the meat once or twice while marinating.
When ready to cook, thread the marinated steak pieces onto skewers alternating with zucchini rings and onion pieces.
Preheat the grill to medium-high heat and oil the grates. Grill the kebabs 8–10 minutes, turning every 2–3 minutes, until the steak reaches your desired doneness.
Remove the kebabs from the grill and let them rest at least 5 minutes before serving.