In a 1-quart or larger heavy plastic food bag, combine soy sauce, vinegar, oil, garlic, and cayenne.
Trim excess fat from steak. Rinse meat and pat dry. Add steak to marinade. Seal bag and set in a bowl. Chill at least 1 hour or up to 1 day, turning occasionally.
Lift steak from bag; save marinade. Lay steak on a grill above a solid bed of hot coals or over high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds). Close lid on gas grill. Cook, turning once, until steak is browned and rare to medium-rare in the center of thickest part (cut to test), 8 to 10 minutes total. Transfer to a carving board. Let rest about 5 minutes.
Pour marinade into a 1- to 1 1/2-quart pan. Bring to a boil over high heat. Cut meat diagonally across grain into thin slices and serve with marinade.