I had a problem like this once with a client site - the host had introduced a limit on the disk space the client could use, and he had hit that limit so could not download the full update package.
My VM upgrade routine now is to use the Joomla install manager rather than the update manager, and simply drag and drop the full package into the upload window. So for the current version, I have dropped the com_virtuemart.3.8.4.10335_package_or_extract.zip package into the installer. All updated in one action, and (so far) I have not had a problem with this. I do backup the site first, though, as we should all do when updating. anything.