Hmm, how would you feel about using “webmail.domain.com” redirecting to your central webmail installation?
Virtualmin already provides a simple way for that to work.
You can configure that redirect in System Settings -> Server Templates -> Default -> Apache Website, and there you can configure “URL for webmail redirect”.
Templates generally only affect newly created virtual servers (or features being turned on). To modify an existing one, after changing the template, you either need to turn the Apache feature off and on, or edit its virtual host file manually.
The webmail redirect is achieved through mod_rewrite rules by Virtualmin, it might be possible to put those directives in an .htaccess too. But the “cleaner” way is to do it the Virtualmin way, as outlined above. Otherwise you’ll probably run into issues if you make other changes to the virtual server, since Virtualmin doesn’t know about your manual thing in .htaccess.