Stir together vinegar, soy sauce, Worcestershire sauce, olive oil, garlic, salt, and pepper. Divide in half.
Add half to re-sealable bag with flank steak. Marinate, refrigerated for at least 1 hour and up to 24 hours.
Add remaining marinade to bowl. Toss with onions and red pepper.
Place flank steak on greased grill over medium high heat; close grill and grill, turning once, until medium-rare, about 10 minutes.
Transfer to cutting board and tent with foil. Let stand for 5 minutes. Slice across the grain.
Meanwhile, remove onions and peppers from marinade. Grill for 5 to 7 minutes over medium-heat until tender and slightly charred. Slice pepper into strips.
To make Chipotle Mayo: Mix together mayonnaise and chipotle pepper.
Spread 1 tablespoon on each bun. Top with flank steak, grilled onions and peppers.