Use Redirect to force users to connect to the secure URL.
<VirtualHost *:80>
ServerName example.com
SSLProxyEngine on
Redirect permanent / https://secure_example.com/
</VirtualHost>
The rest of the configuration can be put in the ssl virtual host (port 443) since everything is redirected.
<VirtualHost _default_:443>
ServerName secure_example.com
ServerAdmin [email protected]
DocumentRoot /var/www/domains/secure_example.com/html
ErrorLog /var/log/secure_example.com/error.log
CustomLog /var/log/secure_example.com/access.log common
SSLEngine On
...
</VirtualHost>