On my XenForo installation, there is only a standard sidebar on the forum home page. If VaultWiki followed this standard, there would only be a sidebar on the wiki index. Adhering to that standard would make VaultWiki's sidebar effectively pointless.
When the popular [bd] Widget Framework is enabled, the "standard" XenForo sidebar appears on all XenForo pages, including the standard XenForo post editor.
Thus there is not really a standard to follow for consistency regarding the sidebar. In fact, I could also argue that if this were "fixed", then some wiki tabs having a sidebar and others not having one could also be considered inconsistent.
In fact, there is a reason that the sidebar appears on all wiki tabs, and that was to achieve a consistent look whenever the regressed VaultWiki 3 feature of "AJAX tab loading" is fixed.