As far as speeding the site up, the best way to do that is get rid of vbulletin all together. While it has served you well these years. There is now to many people for it to handle. You need an SQL server and database. Use one blazing fast computer for this, raid drive, and only use it for message board. It will drastically speed up your site. From what i gather the message board is the only thing slowing you down. My company specializes in database solutions. We have also done work for several large corporations.
This is our company's web site - working on a flash version at the moment.