Like Webmin and Usermin, Virtualmin has an API for writing plug-in modules. It is somewhat different, however, in that Virtualmin modules are also Webmin modules but they also tie directly to Virtualmin domains, so that when a domain is modified, the third party modules will also be modified. Virtualmin modules also have the ability to check for configured Virtualmin features when being installed, to insure compatibility with the system.
The module API is documented at the[a href="http://www.webmin.com/modules-virtualmin.html">Writing Virtualmin Modules</a> page. Virtualmin modules also have access to the Webmin API, which provides a wide range of useful utility functions, so you'll also want to consult the[a href="http://www.webmin.com/modules-index.html">Writing Webmin Modules</a> documentation, as well.
There are also a number of plug-in modules available for reference from Jamie and Vernon Spangler that can be located using the[a href="http://webmin.thirdpartymodules.com/?page=Virtualmin">Virtualmin category</a> at[a href="http://webmin.thirdpartymodules.com">ThirdPartyModules.com</a>. This site is an excellent resource for other Webmin needs, as well, though the quality and completness of modules varies widely.