VB hired another firm to make that app, and it is exclusive to VBulletin.com's forums. You can now PAY them money to have a custom app built for your site and pay who knows how much to have an SN iphone app in the app store. Don't think its worth it and VB customers are pissed that they just didnt make an app that would plug in to any Forum...
Since they got by Internet Brands they have went to hell. VB4 is a joke and why most of us have not switched to it.