...By night they leave their graves. ...crawling, shambling thru empy streets... whimpering, pleading begging for his blood! The Last Man on Earth
Vincent is not some vampire hunter or vampire. He is in fact a genetically modified human being that lives much longer than the average human. He may, in fact, be immortal. At least he claims to be, telling Red XIII that they will meet every year (even after the others are gone) until Red has a family of his own. Vincent, physically, twenty-seven is well over that but may never age. He has not aged a day in the thirty years he slept in the Nibelheim basement and probably won't ever. This couples with a missing past. A lot of Vincent's basic information is unknown, as it will remain. He was born on October 13 to Grimoire Valentine in an unknown or otherwise unstated town. I honestly wouldn't be surprised if Vincent didn't remember it or Shinra had done away with it. Much of his past and childhood is left completely unstated. At twenty-seven Vincent of the Turks was detailed to the scientists Hojo and Lucrecia in the small mountain town of Nibelheim. He states that it was the start of his nightmares.
Thirty years later, when Vincent resurfaced, he met with Cloud Strife and his party in Nibelheim basements. After a reluctant discussion and swapping of information, Vincent finally joined. He tends to be cold and aloof, often keeping to himself. He doesn't care how people go about things so long as they are smart about it as we can see when the party confronts Don Corneo in Wutai. Vincent states, "I don't care so much what you're doing so much as the idiotic way you're doing it." For me, it's not so much the act that's happening as much as the way its being done (leaving a trail, causing lots of trouble for people that can kill you, to name a few things). I think that Vincent wouldn't be the type of guy to meddle in your affairs, a sort of live and let live kind of thing. He dislikes being bothered but when he relies on someone, it is with every inch of his being. This is apparent when he calls Cloud at the end of Advent Children and demands that Cloud talk to Yuffie on his behalf stating that she has no right to call his phone. Although sometimes reluctant he is a trustworthy friend. Reeve takes advantage of this in Dirge of Cerberus, and Vincent willing defends the man going so far as to avenge him, yell at him, and give him a pep talk when Reeve is at his lowest.
An optional character within the original Final Fantasy VII, Vincent holds key information about the series' main antagonist, Sephiroth. Finding Vincent is presented as a game; Cloud has to collect four numbers for the combination to the safe in the second floor study. After defeating another experiment, the group heads to the basement where they learn part of Cloud's past and more importantly Sephiroth's origin. It has always bothered me quite a bit that something so important is left to a character that you may never find. On closer consideration, if Vincent truly didn't want to be found, he wouldn't have been. I tend to think that the Turks were trained to "disappear" if they absolutely needed too. I think more than wanting to disappear; Vincent wanted a way to atone for what had happened. Eventually someone would find him, they would have too. Secrets stay hidden so long as the dead don't talk, Vincent couldn't die though. He already felt his sins were too much, but he thought he might be able to help. Most likely because the case was Sephiroth. At the very least, perhaps he thought to save Lucrecia's son. Maybe he finally decided he no longer had anything to lose.