Well, this is no change - his original abilities in FF7 were the same form-shifting beast transformation abilities. Yeah, his style does seem somewhat vampiric (dark clad, doesn't like light, you find him in a coffin), but it's more about mad "science" than the occult. He was a test subject for the experimental DNA and Mako serums of Hojo and Shinra. This caused his form-shifting, as well as his other less-than-human abilities.
Well I knew he wasn't a Vamp. Thought he was a Werewolf of sorts (shapeshifter if you like that word better)since one of his specials was indeed doing just that. And because sunlight doesn't bother him and he doesn't need to or ever suck blood to stay healthy....I never understood why people thought he was a vampire. He's out in the sun for a lot of the game with your characters. I just assumed the box you found him in just happened to look like a coffin on the Playstation, but was more or less...just an industrial wooden crate of sorts.
If you've seen Vincent's special cutscenes in the hidden cave on Disc 2 or 3 you know that that's not true at all.
Vincent was a member of the Turks, and fell in love with Shinra scientist Lucrecia. Lucrecia however, was more involved with her research (the same research of Dr. Gast and Hojo, involving JENOVA cells, mako and such). Lucrecia was impregnated, presumably by Hojo for the explicit intent of creating an experimental life form made from large amounts of JENOVA cells. This child was Sephiroth. Lucrecia later developed side effects and died, but because of her exposure to the JENOVA cells, she could not assimilate into the life stream and her "spirit" resides in the hidden cave. Vincent feels guilt and remorse for not stopping the experiments - which led to both the death of his beloved Lucrecia, and to the birth of the evil Sephiroth.
Now, I understand why people don't know certain details, cause it's easy to miss certain scenes, like these Vincent scenes, or the also very important Cloud/Zack scenes you see when you return to the Shinra Mansion in Nibelheim late in the game. However, it always shocks me to see how little people understand the plot, even though it's fairly simple and straightforward.
maybe its one of those games you have to play more than once to fully understand. i couldn't say since i've played it many times, but there are some details that might not be picked up with one play through. in fact, that can be said with many rpgs.
on a side note, i didn't even know about the cloud/zack one until i messed around in the debug room.