We do not test on EOL distributions, and we consider our target Perl version to be the version in the oldest distribution we support (at the moment, that is Perl 5.16 in RHEL/CentOS 7). If Debian 8 has a Perl that is 5.16 or newer, Webmin will probably work fine.
Virtualmin will very likely no longer install on Debian 8 using the automated install process. So, you shouldn’t expect Virtualmin to be easy to run on Debian 8.