Dear Ferdinant,
Thank you for the version numbers. The problem is that you are using a terribly outdated version of Virtuemart (which is also known to contain severe security issues, which make it easy for any attacker to hack your site:
virtuemart.net/news/latest-news/462-secu...m2-6-10-and-vm2-9-9b
virtuemart.net/news/latest-news/446-impo...at-joomladay-germany
).
That old version of VirtueMart does not provide the vRequest helper class yet, so the plugin does not work.
My urgent suggestion would be that you upgrade your installation as soon as possible, because you can be easily hacked. If that is not an option for you (at your own risk!), then you can modify the plugin file /home/camraco/public_html/plugins/vmcustom/downloads_for_sale/downloads_for_sale.php on line 253 and change the code from:
$field_id = vRequest::getInt('customfield_id',0);
to
$field_id = JRequest::getInt('customfield_id',0);
That code (JRequest instead of vRequest should work on Joomla2, but it will no longer work with Joomla 3. For that reason, VM added its own vRequest class, but as I said you are using an outdated version of VM.
Please bear in mind that you'll have to make that modification whenever you reinstall/upgrade the downloads for sale plugin.
You'll also have to make the same changes a bit further down in the file (around line 276 are some more calls to vRequest)
Best regards,
Reinhold