Note: Appearance issues are often caused by caching plugins. When Posting Please Include Your URL. Search Forum Using Google Search Box Above IMPORTANT: If after forum login, you appear logged out, click on "New Posts". You should show as logged in then. It's a Vbulletin bug.
Assuming your existing blog is based on WordPress you should be good to go. Remember that if you have a caching plug-in installed on the blog, you should deactivate it while the site is in transition or you won't see your changes as you make them.