Completed for RC 4. That is, as of RC 4, VaultWiki should store the ID of the appropriate previous revision for each new revision. This is not simply the ID of the previous row, but rather, the ID of the current revision at the time the edit was started. This is different to the flawed VaultWiki implementation, because there can now be soft-deleted and unapproved edits (and in 4.1, other kinds of edits) in between a revision and its parent.
The upgrade script does its best to guess this for existing revisions, though there will be inaccuracies in some cases. This is because deletion and approval states usually changed after an edit was originally made, but the script cannot determine when a state-change occurred (this is not a recorded metric).