This is fixed on your site and will also be fixed in the next release. The deferred task I was talking about (when the VaultWiki add-on is re-enabled is supposed to generate missing user fields for users who joined while the add-on was disabled) had been marking itself as complete without actually doing any work. After fixing the issue, re-disabling-and-enabling the add-on, the task did its work; I used the rebuild contributions tool; and now the user mentioned in the first post shows contributions on his profile.
Marking this as Fixed. Thank you for bringing this to our attention.