We are trying to update from one build to another and we are getting errors in the installation utility saying the tmp/install_4e455bae4e636/install.vmallinone.php is missing. This file is indeed gone. Our entire tmp directory is empty and must of been emptied by an admin during our site move.
How can we manually upgrade our installation or from RC2 (current) to RC2b?
I suppose you are using Joomla 1.7. In this case I would suggest to wait a few days for the next release, because meanwhile there have been many changes again to make it work better with Joomla 1.7. When the next VM2 RC is released, you can export your database first with phpmyadmin and then try the VM2 live update. If that fails for some reason you can uninstall VM2, clear all folders from VM2 files, delete (drop) the VM2 database tables and install a brand new VM2. After that you can use phpmyadmin again to import the database from your current version.
You may try http://dev.virtuemart.net/projects/virtuemart/wiki/Manual_installationupdate_of_VM2