Cut steak diagonally across grain into thin slices; set aside. Combine vinegar and next 5 ingredients in a large zip-top plastic bag; add steak, and seal bag. Let stand 10 minutes.
Heat a large nonstick skillet over medium-high heat; add steak and marinade. Cook 4 minutes or until browned. Add bok choy; sauté 3 minutes or until tender. Sprinkle with sesame seeds.