Are you using a default style?
To create a default style, 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
Set this as the default in AdminCP > Settings > Options > Styles & Languages > Set Default Style. Then go to Maintenance > Clear System Cache. Then try to edit the article using the totally default vbulletin style - do you still have the same problem?