hi,
You can safely apply the 2nd solution that was explained here:
www.flexiblewebdesign.com/forum-virtuemart-template-support/2763-add-to-cart-500-error#12013
Because it is not "hack", you are not hacking/modifying any file, you are just keep using the old official files (only for 2 files). None of the VM3.x updates were released for security. So there is no security problem by keep the old version for 2 files. You don't need to be skeptical.
You can check the Virtuemart release history for more info.
Regards.