Currently the Visitor Generator uses manual call to the DB, instead we should demonstrate best practise and use the recently introduced API to force old reports to be reprocessed, see: http://piwik.org/faq/how-to/#faq_155
Code to replace is in: /trunk/plugins/VisitorGenerator/Controller.php at L72
In b613d35: Use the Piwik API instead of directly accessing the database.
In 011794a: Check for browser archiving in Controller.php
Show text according to browser archiving setting.
@capedfuzz, hope you didn't invest much time in this. Needed the VisitorGenerator for Alerts so I took a few minutes to refactor it.