Hacked through provider - files added.

[Dave]

Try this: login into the admin cp > Maintenance > Diagnostics > Suspect File Versions. That will display any files which do not belong to vBulletin or are modified. Also check your plugins at Plugins & Products > Plugin Manager. There might be some fishy plugins with backdoor code.

In case you use shared hosting, they probably "rooted" the server and ran a script to replace all index files of all websites with their deface page. If that's the case, you better find a completely different host.