DocumentRoot /webhosts/
[...]
<VirtualHost *>
ServerAlias www.yourdomain.com
ServerName www.yourdomain.com
RewriteEngine  on
RewriteCond    %{HTTP_HOST}  ^yourdomain.com
RewriteRule    ^(.*)$        /www/$1 [L]
RewriteCond    %{HTTP_HOST}  ^www.*
RewriteRule    ^(.*)$        /www/$1 [L]
RewriteCond    %{HTTP_HOST}  ^(.*)\.yourdomain\.com
RewriteRule    ^(.*)$        /%1/$1 [L]
</VirtualHost>