According to the Virtualmin Installation FAQ:
What if I already installed Apache, Postfic, MySQL, Dovecot, etc. on my system?
If you installed from the OS standard repository for your OS,
everything should be fine. If you installed from any third party
sources, or from source, the installation will fail and things will go
badly. The install script cannot accommodate packages installed from
non-standard sources. It just isn't that smart.
If you can re-install your OS, it is recommended that you start with a
freshly installed Grade A supported Operating System.
And this is from Webmin FAQ:
What effect will Webmin have on my existing configuration files?
Just installing Webmin will not cause any config file changes to be
made. When you start to use it, only the config files related to the
changes that you make in Webmin will be modified. For example, using
the Apache Webserver module would not effect your Sendmail
(Virtualmin is an addition to webmin, not part of it, so sometimes webmin faq may be incorrect for virtualmin)
My advice based on the above:
If you can, (even if that require some work), "start with a freshly installed Grade A supported Operating System".
If you can't, make a backup of your system (at least, backup all your configuration files and the state of the installed packages) and install Virtualmin.