Ok..let me break it down
vBulletin is styled by skins. The problem with this is most if not all skins you can download modify the stock templates for the system. Doesn't sound bad but when you go to upgrade the forum you're left with 2 choices afterwards. Edit the templates from the skin (with a 99% chance of losing any custom images, links and coding unless you hand edit each one which takes time..a lot.) or Leave them be and run the possibility of new features, security issues etc. not working.
From my experience in Administrating forums and general knowledge of vBulletin the best option is to just take this current blue style and simply edit the color and make it a new style. The blue will still be availible of course, selectible at the bottom of the forum.