I'm also having this problem, and it is driving me crazy:
* I have the debug by IP address fixes mentioned above
* I can see the errors in the file logs, but they are not displayed
* Non-VirtueMart originating error messages such as login errors display fine, so I know the template is OK
* I can use default templates, or custom ones, the result is the same (nothing displayed)
* All templates have <jdoc:include type="message" /> in them
* To the degree I can debug things, the error messages seem to be there, but following the error display logic is quite difficult.
Any find a solution to this?