Styles & Templates > Style Manager > Add New Style
Parent Style: No Parent Style
Title: Default vBulletin
Allow User Selection: Yes
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Then browse the site using that totally default vbulletin style - do you still have the same problem? If not, I would guess it is a problem with your custom FORUMHOME template (or header, headinclude, navbar, or footer template)
One of your plugins is causing this, might be one of your ads, this happened to me when I was using a rotating ads plugin and it created a javascript conflict which caused my sidebar to drop to the bottom.