I have not done custom themes for a forum - but i have for Coppermine photo galleries, and I would have thought tasks were similar.
Basically, producing any page as a photo-shop theme is usually a big no-no. Individual users will tweak their browsers to suit themselves, others will have accessibility requirements. And making the page search-engine friendly would be a nightmare, IMO. If you are, none the less, hell-bent on using your photo-shop layout, then you can either use a table, or, better, use CSS with absolute positioning.
I should think the v bulletin documentation will help you with how to introduce your own theme or skin. And I would hope it has its own support forum, which will be best-placed to give you further advice..
 The above comment was written before this thread was moved from the HTML section to this - which I must admit I didn't know existed. Oops. [/edit]