In a bowl, combine soy sauce, garlic, ginger, vinegar, and oyster sauce. Add sliced beef and marinate for at least 15 minutes.
Heat sesame oil in a large skillet or wok over medium-high heat. Add beef and cook for 2-3 minutes until browned but tender. Remove and set aside.
In the same skillet, add broccoli, zucchini, and bell peppers. Stir-fry for 4-5 minutes until crisp-tender.
Return beef to the skillet, toss with veggies, and cook another 1-2 minutes. Top with green onions before serving.