vBulletin does not have an inheritable color palette system like XenForo does. It is not possible to use existing vBulletin CSS colors without lots of style conflicts. In early versions of VaultWiki 4, it was designed like your request, and we updated it to use new style variables because of all the problems that existed with the other way.
You should not have to modify CSS to get VaultWiki to match your site's theme. VaultWiki includes Style Variables for the various colors it uses that you can change in vBulletin's Style Variable manager. With vBulletin's Style Variable manager, you should be able to change them all in one submission.