Hello,
This is a virtuemart problem, it has nothing to do with the Virtuemart Registration Redirect plugin. You would have the issue you are having whether the plugin was installed or not.
So, you should take this issue to the VirtueMart forum.
Here is what I would try, which is just a suggestion, no guarantee that it would work. I would try making a menu item for VM registration page, that has public access. You will need to make the patch to the KC Registration Redirect plugin suggested here:
Remeber ItemID. See if that will allow you to register without the access error. If that doesn't work, then ask on the Virtuemart forum for help.
Good luck.