This occurred because an operation took longer than the MySQL server's connection timeout, and then attempted to perform a MySQL query. Unfortunately there is no truly reliable way to determine how long an operation will take before performing it, and vBulletin doesn't automatically refresh database connections on long-lived scripts. For now, the issue can be worked around by simply retrying the operation. In extreme circumstances, we might need to isolate the specific page ID with edits to the importer files and limit what is imported about it.
This occurs because the amount of time taken to process any one wiki page is related to the length and complexity of that page's content. It is possible that any single page by itself exceeds timeouts even if you just attempt to view that page; this is one vulnerability of VaultWiki 3.x, which basically allowed users to modify pages in such a way that the page is no longer accessible or modifiable (denial of service). Once you are upgraded, VaultWiki 4.x provides a number of settings that will prevent users from being able to do this moving forward.