Could I skip the alias part and enter: ServerName www.subdomain.domain.com subdomain.domain.com subdomain.altdomain.com www.subdomain.altdomain.com
No. You get one ServerName directive, but you can have multiple ServerAlias entries. See http://httpd.apache.org/docs/2.0/vhosts/name-based.html and http://httpd.apache.org/docs/2.0/mod/core.html#serveralias