Heat vegetable oil in a dutch oven over medium-high heat. Brown the flank steak on each side, about 4 minutes per side.
Pour in the beef broth and tomato paste, then add the onion, bell pepper, garlic, tomato paste, cumin, cilantro, olive oil and vinegar. Stir until well blended. Cover, and cook on 350 degrees for 4 hours.
Remove the meat to a cutting board and let cool enough until you can handle it. Shred the meat by hand or use a couple of forks. To have shorter strands I cut across the grain at two inch intervals before shredding.
Put the shredded meat back in the cooking liquid to heat back up then serve over rice.