What's the deal with this guy? He's a vampire and a count, so how can he expect to live as a count and also a vampire for a prolonged period of time? Eventually people will start to ask questions. What will he do when people are wondering why he isn't dead after 100 years?

Does anyone think that Skyrim will answer a few questions about what happened to him between the time of Oblivion and Skyrim?

His alibi is that he's a powerful Breton wizard. Powerful wizards can live for very long periods of time. Bretons, in general, have longer lifespans than the other human races.

I don't expect to hear anything about Janus Hassildor in Skyrim.

The Council of Mages is aware that he is a vampire and have an 'arrangement' with him. I suspect the Elder Council is aware, with a similar understanding. As far as his subjects, he has a reputation as a reclusive wizard. I expect those subjects with excessive questions will be introduced to his dungeon - and they probably know it. It ain't a democracy and as long as he retains the loyalty of his guards and the tolerance of the Elder Council, he is probably not terribly concerned about the opinions of his subjects. Just a thought.

There's a reason that Castle Skingrad is outside the city walls.

He is actually an Imperial. With an Imperial name. A loading screen does say that he is believed to be a powerful Wizard.

He's reclusive so few people actually interact with him. You've seen in the Information at a Price quest, that he just takes care of any possible problems before they get too close. And as mentioned above, any one asking too many questions probably gets thrown in a dungeon or otherwise "dealt with". He is also respected as a very competent count, in that he runs Skingrad well, so Skingrad citizens would likely not want to "make waves", or they could end up like Bravil. All that and with support from the Council of Mages, he is able to get by.

I'd just like to note that he's an Imperial, not a Breton. Another interesting thing is that despite having the repuation of being a powerful wizard, his actual class is Noble so he can't cast any spells.

Still, if Telvanni wizards can live for thousands of years it should be possible for a human sorcerer to live for a few centuries. Long enough for people to forget exactly how long he's been alive.