Sorry, I wasn´t clear enough.
I didn´t create a virtualhost specifically for webmail. I created a vhost so that I can my server from the Internet via my registered servername from dyndns (https://nextcloud.mydomain.tld). Additionally I want to access webmail via this virtualhost too (via https://nextcloud.mydomain.tld/webmail).
I didn´t configure anything Special in the vhost, except I created a symbolic link to the nextcloud Directory /var/WWW/html/nextcloud in /var/lib/nethserver/vhost/nextcloud (Nextcloud in new virtualhost)