Place flank steak in a glass dish. Combine the next nine ingredients; pour over meat. Cover and refrigerate 4 hours or overnight. Drain and discard marinade. Grill meat over hot coals until it reaches desired doneness, about 4 minutes per side for medium, 5 minutes per side for medium-well. Meanwhile, in a skillet, saute onion and peppers in oil for 3-4 minutes or until crisp-tender. Slice meat into thin strips across the grain; place on tortillas. Top with vegetables; roll up and serve immediately.