Another option that I use to use at work is to click on the "Show Printable Version" in the thread tools section of whatever page you are trying to view.
Edit: Upon further research, vBulletin does not currently support a mobile-friendly browser option. vBulletin 4.0 should be out soon but I'm not sure if that version will support it either.
So the best way to get a mobile-friendly mode is to disable all images, avatars, and sigs, or use the printable option listed above.