Vetinari wasn't from Uberwald, he simply visited it in his youth. Given that he was in Twurpe's Peerage, and was qualified to become Patrician, one would assume he was one of the Lords in Anhk-Morpork. It's highly unlikely that he's a vampire, since there are so many ways of sussing the bastards out. Werewolves, including Angua, would be able to smell him out. Other vampires would know. And a vampire wouldn't be harmed by arsenic.

Speaking of vampires, the reason the turtle had no effect was that the vampires had trained themselves so as not to be harmed by any holy symbols. They had a great deal of trouble at the end when they started losing this ability. Presumably, if Mightily Oats had presented them with his turtle at that point, they would have been repelled by it. Unfortunately, he'd lost it by then, and had to make do with a very sharp ax.

EDIT: Ba should point out that he's aware that nobles of other lands are also in Twurpe's Peerage. He's just pointing out that Vetinari was firmly of the upper classes. That he became Patrician points very strongly to Anhk-Morpork origin.