I doubt it is the skin. You can check by doing 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?
However, if you have it all turned off, and you add something in the "reason" text area, the Edited By *will* show up no matter what you have set for the option.