Have you tested this since updating to 4.0.17?
VaultWiki did not support username changes or user merges prior to 4.0.17. Starting with 4.0.17, VaultWiki automatically changes columns like vw_revision.username whenever a username-change action is perfomed, provided that XenForo's setting AdminCP > Home > Options > Performance > Update denormalized user names on user name change = On.
If you were checking this on the wiki index, please note that the wiki index might be serving a cached version of the page from before the username change. Try adding &nocache=1 to the URL to see an uncached version of the index.