Under XenForo, the vw_commentbit_vertical template is simply a wrapper for XenForo's message template. XenForo's message template provides all layout. If your message template has been modified to support horizontal layout, then it would automatically be applied to wiki comments as well. There is no need for a separate vw_commentbit_horizontal template, because under this principle, it would just be a clone of vw_commentbit_vertical (invoking message with some variable names set).
But your message template contains specific references to 'thread' and 'post' elements, which is improper, because as you already noticed (from what I see in the message-template code) this causes problems in other content types, such as conversations and tickets (and of course VaultWiki comments). As described in my post above, these 'post'/'thread' references should be isolated within the "post" template (which wraps the message template too), or other such template depending on the content-type, and passed in as a generic variable. It may well be a bug in the underlying XenBase style as you suggest, or it may be a customization you made to the style. Either way it is not a bug that VaultWiki has the power to fix. The bug is not present in our code, nor possible to workaround in our code without writing a completely unique message template for VaultWiki (an approach that the XenForo devs did not intend add-on authors to take).