This is not a bug. The wiki style matches the default vBulletin style. If you have a custom style then you also need to customize VaultWiki.
This is not true for vBulletin 4. In vBulletin 4, VaultWiki is able to use vBulletin's stylevar system to determine the correct look. In vBulletin 3, most forum colors are not stored in stylevars.