What do you have set at vboptions > message attachment options > use image lightbox? I don't know why it wouldn't work for hover. Maybe change it > save > change it back and see if it works.
Also, you can try this:
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?