Heat the grapeseed oil in a large skillet over medium-high heat. Sear the steak, about 4 minutes per side.
Transfer the beef and juices to a medium-sized, heavy-bottomed pot. Cover with the beef broth and stir in the tomatoes, onion, bell pepper, and garlic along with the cumin, salt, cilantro, olive oil, and vinegar. Cook, covered, over medium heat for 45 to 60 minutes, until the meat is tender.
When ready to serve, shred the meat with forks and place on top of mixed lettuce leaves.