In a 6-quart slow cooker, stir together the tomatoes, bell pepper, shallots, olives, capers, garlic, oregano, cumin, black pepper, 1/2 tsp. salt, and the bay leaves.
Cut the flank steak in half with the grain, then cut each of these halves into thirds across the grain. Nestle the pieces of beef into the sauce.
Cover and cook on low for 8 hours, or until the beef can be shredded easily.
Discard the bay leaves. Shred the meat in the cooker with two forks, then stir in the cilantro, vinegar, and salt to taste.
The beef can stay well on a keep-warm setting for up to 3 hours; shred it and stir in the cilantro and vinegar just before serving.