Sprinkle one side of roast with 1 tsp. meat tenderizer. Pierce deeply with a fork and turn over. Repeat with other side of roast. Let stand 30 minutes.
In a 2 qt. glass baking dish, combine vinegar, oil, lemon juice, onion, thyme and bay leaf. Add roast and turn to coat.
Cover with plastic wrap and refrigerate at least six hours or overnight, turning roast and spooning marinade over it periodically. Light charcoal in outdoor grill. Discard marinade and scrape onion and herbs off meat to prevent them from burning when grilled.
Sprinkle meat with pepper and place in same dish. Cover again and microwave on high 5 minutes. Turn roast over and microwave on high 5 minutes more.
Immediately, grill over hot coals for 30 to 30 minutes for medium doneness. Slice meat diagonally, serving juices with meat.