Kirito is a powerful swordsman so taking on a shadow creature like Vaatu should be easy. Vaaty is no slouch when it comes to a fight so I’m not underestimating him, but Kirito is a lot faster and he’s also a lot more determined to win. Overcoming his fighting spirit is just not something that I see happening. He’ll keep chipping away at Vaatu’s HP until he’s down for the count. Kirito wins.