Howdy,
The following has a list of the command line Perl tools for backing up virtual servers:
http://www.virtualmin.com/component/option,com_openwiki/Itemid,48/
id,virtualmin_cat_backup_and_restore/You could certainly call those from within a Bash script. Simply play around with it on the command line until you have it backing up the domains and features you want, then just put that exact command line into a bash script, and you'll be all set.
If you had other Webmin/Virtualmin features you wanted to access, the full list of available commands is here:
http://www.virtualmin.com/documentation/id,command_line_api/Have a good one,
-Eric