There are multiple ways to cause this:
- You manually removed the vw_permission table while VaultWiki was enabled
- You uninstalled VaultWiki while VaultWiki was enabled, and a user visited the forum at the same time that VaultWiki was uninstalling.
It is not possible to resolve either situation at the software level 100% of the time.
For the second cause, it is best to always disable an add-on and wait a few seconds before uninstalling it in order to avoid race conditions like this. This issue can occur with any add-on.