Simply put, Vaas is just like Jason. Both of them were drawn in by power. Hoyt recruited Vaas using drugs and the promise of power. Citra drew Jason in using very similar tactics, using drugs to cause hallucinations that Jason believed woud lead him to become a powerful warrior. Basically both of them are mere pawns in a deadly game of chess on Rook island. Hoyt uses Vaas to terrorize the isand, and Citra uses Jason to fight back at the pirates. When it comes to conflict, Jason and Vaas lead the fight, they are the ones doing the main fighting. Hoyt and Citra just give orders and relish in the victory that their soldiers earn.
Vaas seems somewhat aware of Citra's true plans for Jason, as he mocks the idea of Jason becoming an all mighty warrior just because Citra says she can make him one. The idea at the start is that Vaas is insane, but when you compare that to the fact that Jason has deluded himself into thinking he is some kind of savior ,then Jason seems even more insane.
Vaas has been compared to the Cheshire Cat, in the way that he almost acts as a guide. He is Jason's motivation for vengeance, he is the constant threat just waiting to tie a block of stone to your feet and drown you, he is the one that spared your life so that you could become just as brutal as him. In a strange way, he made Jason become just like himself, brutal and uncompromising. He acts as a warning to Jason because Vaas is what Jason could become, a pawn that acts as someones attack dog. During the final encounter with Vaas, he mentions being reborn in a Christ like way. This is because he lives on inside Jason, as Jason has now become just as brutal and prone to violence as he was.
Prior to the final encounter with Vaas, Jason climbs out of a mass grave, and the way he has to push his way out is almost like he is being born again after almost being killed by Vaas (a lighter in Jasons pocket stopped a bullet Vaas fired at point blank range. Dennis then says you have returned from the dead which also leads into the whole being reborn thing. Its after this incident that Jason finally learns that he has to become just like Vaas to beat Vaas. He assaults Vaas directly, infiltrating his island and slaying everyone there, including Vaas.
Vaas also claims to have set you free, by allowing to unleash a brutal instinct. Its like Vaas in example of everything Jason needs to be in order to "win". Vaas has a very interesting presence on the island. I almost expected him to be a part of Jason's Mental State seeing how he is barely seen. Even when Vaas died I still felt his presence on the island, probably because Jason has now become just as notorious as Vaas after killing him. Its like he has killed the king and taken his throne.
