Solution 1:

Try this:

<VirtualHost *:80>
    ServerName example.org
    ServerAlias *.example.org
    RewriteEngine On
    RewriteCond %{HTTP_HOST} ^(.*\.)?example\.org$
    RewriteRule ^/(.*)$ http://%1example.com/$1 [R=301,L]
</VirtualHost>

..then have your other vhost (the one serving the content for example.com) configured with ServerName example.com and ServerAlias *.example.com.

Edit: made the matching of subdomains optional by adding the "?" quantifier