ServerAlias *.sub.example.com
 
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www\.)?(.+)\.sub\.example\.net 
RewriteCond /Users/username/Sites/sub/%2/httpdocs -d
RewriteRule ^(.*) /%2/httpdocs/$1 [L]

ServerAlias *.sub.example.com
 
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www\.)?(.+)\.sub\.example\.net 
RewriteCond /Users/username/Sites/sub/%2/httpdocs -d
RewriteRule ^(.*) /%2/httpdocs/$1 [N]

Add to vhost block. Dynamically maps all subdomains of sub.example.com to their respective document root. Note: Doesn't work great when the sub sub needs to add additional mod_rewrite rules of its own, i.e., a Drupal installation.

Mod_rewrite to dynamically map subdomain directories.

Apache