Combine steak, onion and capsicum in a shallow baking dish. Whisk together garlic, lime juice, oil, vinegar, cumin, salt, pepper and chilli in a small bowl. Pour over steak mixture; toss to coat. Refrigerate, covered, for 1 to 2 hours.
Preheat grill. Cook steak about 10 cm from heat, 2 minutes each side. Add the onion and capsicum. Spoon any remaining marinade over steak and vegetables. Grill, turning meat and vegetables occasionally, until the meat is cooked as liked and vegetables are crisp-tender, about a further 7 minutes. Leave meat to stand 5 minutes.
Cut meat diagonally across grain into thin slices. Divide meat, onion and capsicum evenly between warmed tortillas. Top with cheese. Place under grill just until the cheese melts, about 30 seconds. Fold tortillas over filling and serve.