They work just fine for me.
edit: Whoops, I thought you were talking about the individual expand/collapse buttons, not the main one.
Create a new style with no parent:
- Styles & Templates > Style Manager > Add New Style
- Parent Style: No Parent Style
- Title: Default vBulletin
- Allow User Selection: Yes
- Save
Then browse the site using that totally default vbulletin style - do you still have the same problem?
another edit: Error in firebug:
NetworkError: 404 Not Found - http://www.domain.com/clientscript/yuianimation/animation-min.js?v=420
So, you are missing the file.