Place flank steak in resealable bag; add soy sauce, rice wine vinegar, cilantro, brown sugar, and sriracha lime seasoning. Seal bag and turn to coat steak with marinade. Refrigerate for at least 3 hours or overnight. Remove flank steak and discard marinade.
Preheat grill to medium-high; grease grates well. Cook steak for 5 to 6 minutes per side or until medium-rare. Let stand for 5 minutes; slice thinly against the grain.
Meanwhile, divide baguette into four portions and slice in half lengthwise. Mix together butter, garlic powder and red pepper; spread over baguette. Grill baguette for 1 to 2 minutes or until lightly toasted.
Toss together arugula, olive oil, vinegar and salt; arrange flank steak and arugula over bottom halves of baguette and cap with baguette tops.
Spread mayonnaise over toasted garlic bread if desired.