Sorta already exists... looks like there are plug-ins a vBulletin based site can install, and then users can download an app that communicates with the plug-in...
Forum Runner - vBulletin / phpBB Forum iPhone App
Tapatalk - Forum App on the iPhone, Android, Nokia, and BlackBerry for vBulletin, phpBB, IPBoard, and SMF
Although... alternatively.. I think an iPhone UI for the site would work too...
Google: "vbulletin iphone theme"