I have the same problem.
In the config > template at 'Vendor Media URL' I changed 'images/stories/virtuemart/vendor/' to 'www/images/stories/virtuemart/vendor/'.
Still the same error
TCPDF ERROR: [Image] Unable to get image: /htdocs/images/stories/virtuemart/vendor/resized/2512f1621d80ff362cad068834f00e411_90x90.gif
The image is there, I can view it with
domain.com/images/stories/virtuemart/vendor/resized/2512f1621d80ff362cad068834f00e411_90x90.gif
and when viewing the invoice as not pdf:
index.php?option=com_virtuemart&view=invoice&layout=invoice.......
I also tried by uploading the image to /htdocs/images/stories/virtuemart/vendor/resized/ (so outside the www-folder) but no change, still that error.