As a short term fix for Let's Encrypt until Virtualmin has full support. This workaround was a tad hidden in a long message chain on the Webmin forum, thought getting it into the open might help a few people.
Centos / Red Hat based : " yum install socat certbot "
Debian / Ubuntu based: " apt-get install socat certbot "
System Prompt ># " certbot register "
Complete with a valid e-mail.
Try to request new Let's Encrypt
I followed these simple steps on Centos 7 - worked perfectly.