Sympathetic may be the wrong word to use, but I found by the end of the game that Vaas seemed to make a lot of sense as a character and a person, and the way in which he was broken, what he does now, just came together in a very real way.
Throughout the game, they like to really amp up the fact that he is a monster (which there is no doubt that that is what he's become), lots of 'fuck you's directed towards him and such, but I found the scene where Oliver, Liza and Jason are all tied into a chair before the building burns to be one of the most humanizing scenes for Vaas, especially in retrospect. The things he says seems like just a pissed off brother who hates outsiders at the time, talking about how Jason has caught jungle fever, but I really felt I understood why he was so angry, having been one of the Rakyat, and having eventually turned his back on them as well.
When he starts to talk about his sister, you can see how he broke. Citra likes to drive home that Hoyt twisted him and broke his mind with drugs and money, but we've seen by the end of the game how the Rakyat operate. Hallucinogenics, both violent and passive coercion, tyranny and primal law. It seems more than anything, that Vaas was a broken man before Hoyt ever came along. He's used very obviously by his sister to accomplish what ends she desired, pushing him beyond his capacity to deal with stress, shock and violence, to a hollow state and a sort of desperate emptiness. When he speaks to you, you can see he once loved his sister, who would appear to be his only family, but that he now despises her, understanding in hindsight the ways that she was twisting him.
You are angry, Jason. You are angry. Ok, I get that... I get it... I mean, without family, who the fuck are we? There was a time where I would do anything for my sister, you know? First time I ever killed was for my sister... But that was not enough for her, nononononono please... You see the thing about our loved ones, right? Our FUCKING loved ones, they come and they BLINDSIDE you every fucking time. So they say to me, they say "Vaas! Vaas! Who the FUCK is it going to be!? Them, or me!? ME, or THEM!?" Hehehehe... Like, you know, like they fucking think that I need to make a fucking choice.
While I came away from this game ultimately a little disappointed as far as an experience goes, I still feel good about having experienced it. Vaas feels like one of the most shockingly well realized characters that I can recall seeing in a video games (ignoring The Walking Dead) to the point where you almost feel sorry for him, what he's become, how suicidal his emptiness has made him and how desperate he seems to fill that void.
You know. Up until the point where you have to find a different description for the above scene than 'the one where he douses you and your friends in gasoline before setting you ablaze'