This is probably documented somewhere obvious where I can't find it.
The Webmin - Virtualmin backup format is:
A gzipped tar file for each virtual directory.
Then, the subfolders are tarred too.
MySQL: It's burried in the folders in the file format [virtualmin user]_mysql_[database name].gz (I used dir /s *mysql* to find it).
tags: virtualmin, backup, restore