A similar error may occur if a user runs a "New Wiki Articles" search while the add-on VaultWiki is disabled. This can be worked around in the next release by attempting to detect whether the add-on is enabled in the code for each handler.
The problem is XenForo does not automatically disable handlers related to disabled add-ons. I have argued in the past that this is a design flaw in XenForo, specifically when it came up regarding Deferred handlers.
However, in this case it should not be able to find vw_Hard_Core. I don't know why it would ever not be able to find vw_XenForo_Model, as XenForo's own autoloader should get that automatically when it sees that library/vw/XenForo/Model.php exists. If you recently re-uploaded VaultWiki, then it is possible that library/vw/XenForo/Model.php was locked by the upload process at the same time that someone attempted the search. In this case, this is not a bug.