Just making it clear - he was in a sideways stance all the time. That kind that you see in eastern martial arts, not boxing. With such a lean guy as Vos, simply taking a squatting stance will put a few inches between the foot and his stomach, as he will have to bend his chest forawrds to balance, thus pulling the stomach inwards, so he didn't have to move that fast at all. Of course, he was turned slightly towards Sev, but his body wasn't perpendicular to the kick, giving him time and space for this manouver. As for his arm, Vos would still feel that the kick is actualy pushing it backwards, and pulling his own arm out of the way would be possible, since all he has to do is actualy pull his elbow to his side, covering less than an inch of distance to reel his hand out of harm's way.

EDIT: and leaning backwards while using his right leg as a support would move the chest, which would be lower due to the squatting stance, out of the foot's range as well.

P.S. - im not adding this to the list of Vos' powers, but some martial art practioners specificaly train to exert great amount of force over small distances.