to go to your webmail, you first need edit your DNS records to make mail.computervitals.com point at your server IP.
Then configure your httpd.conf for your particular domain to add in that alias for your domain's IP address to point to your webmail directory.
For directadmin, the domain's custom httpd.conf file can usually be found at /usr/local/directadmin/data/users/YOURDOMAIN/httpd.conf.
Just check the mail httpd.conf file located at /etc/httpd/conf to see the include path. After locating your domain's custom httpd.conf file, you need to edit the ServerAlias line to add the info you want.
