From what I understand, Aizen was experimenting with holloification. Also from the show, the Zampaktou is a manifestation of the owner's soul. So a Vizard is one where the evil potential has entered a person and the evil Zampaktou essentially overpowered or ejected the good Zampaktou.

With Ichigo, his soul chain was severed and he had to fight against the evil potential of his soul which was then incarnated through Hichigo. Neither side was able to eject the other, so Ichigo has to maintain control.

So am I to understand that the good Zampaktou of Vizards no longer exist. That Aizen was able to eradicate them in order to induce the Holloification process?

Oh, and do Zampaktou have their own soul chain even though they are manifestations of their owners?

Are ressurection similar to Bankai? If so, does this mean that Ichigo has an evil version of his Bankai up his sleeve that he hasn't used yet?

From what I've seen of the show, Arrancar seem to be the opposites of Shinigami. Ichigo should theoretically have the ability to go resurrection AND Bankai at the exact same time.