Well to be perfectly honest, i dont know much about creating apps but I do have some ideas in mind.
The mobile platform is already very user friendly so something similar to this would be a good starting point as opposed to trying to condense the full sites contents into the app.
The login would the same, leading to the users vbulletin menu just like on the mobile platform. The layout on this page would work very well for most of the application. For instance, when clicking on the 'forums' icon in the vbulletin menu, the different forums would pop up in the same style and swiping right could display more forums in the same 3x3 matrix until the last one.
Individual threads would have to be in a different format of course because there are so many, but a screen with 10 threads per page would work well.