He's keeping his entire body together after it being sliced into hundreds of pieces through pure will power. Every bone in his body is broken yet he still walks and fights.
He drains the force slowly from the people he fights like all Sith Assassins that worked with/under Nihilus do, meaning the longer the fight will last the more powerful Sion will become the weaker Vader will become.
Sion is also the head of a bunch of Sith Assassins so he has quite some power. He used to be a Sith Assassin during the Jedi Civil War. The guy has some skillz.