Just to clean this up into one post.
You are not allowed to remove ANY copyright regardless unless you have the explicit permission to do so. This is for anything related to vBulletin or not. Many software have a 'branding-free' option, which gives you permission to remove the copyright. As for vBulletin, the minimum text must be, unless you have bought a branding-free license,
Code:
vBulletin, Copyright ?2000 - 2007, Jelsoft Enterprises Ltd.
For your skin/vbAdvanced copyright, you will have to check with the author of those products.