Yeah, unfortunately looks like they're going to go with another mindless army of enemies thing rather than one big baddie. From the looks of things, Tony (and possibly Bruce) creates Ultron by accident while attempting to further automate his suits (perhaps he's trying to give Iron Man up without letting The Avengers down or something), and Ultron replicates himself into an army.
Vision is indeed Bettany. Not sure how they'll introduce him (Ultron created him in the comics).