RewriteEngine on
RewriteCond   %{HTTP_HOST}              !^www\.site\.com$
RewriteCond   %{HTTP_HOST}              !^site\.com$
RewriteRule   ^(.+)                     %{HTTP_HOST}$1    [C]
RewriteRule   ^([^.]+)\.site\.com(.*) /path/to/docroot/$1/$2
[download]