You are not receiving an answer because it is not an easy thing to answer. VB5 was designed to handle this such problem. All that is required there is to change the viewport and the widths. The easiest solution is just to provide another URL for your mobile ( ex:
http://yoursite.com/mobile/ ) which runs your mobile script. Then you would shut down the auto-detection script from detecting mobile on your main URL.
You can also just add this to the menu on you mobile script. It might even be there already. Sorry, I do not know, we use our own kludge.
Customizing the vb4 code can of course can be done, but it is a bigger project then you would expect. vb4 was just not designed with adaptable code in mind.
I know most of my friends with their iphones just view the entire site and zoom when needed. They need a good connection and a good data plan for that. I think the difference is from 200-300k for normal vbulletin and 25-35k for the mobile style. So be aware of this.
My company has mostly succeeded in rewriting vbulletin4 for mobile applications, but it was a huge job over time and looking back I can say it wasn't worth it. We would have spent better time building our own mobile app. VB4's mobile app unforunately was too limiting for us.
If you can wait for vb5, this might be your answer.
I am around if you have any other questions.