In VirtueMart we always followed three main principles.
First princip, the folder hierarchie of the svn is the same as in the installer.
What means this? You can just download the SVN, export it and upload it per FTP to override the old files.
The second princip is that we can execute the installer script within the tools of VM.
So if you use your FTP to update the files, just run "Install or if necessary update tables".
The third principle is, that the table updater always updates the tables to the last layout.
If there are things to consider, the updater script takes care of it. So always update to the last version directly.
Tables of plugins are updated saving the configuration.
And before you have to search, here is right svn directory of the last core. https://dev.virtuemart.net/projects/virtuemart/repository/show/trunk/virtuemart
Creating an installer is a bit more complex. The core installer is just build by the folder "adminstrator, "components", "install" . In this case you must copy the "install" folder from /administrator/components/com_virtuemart/ into the zip root directory and add the virtuemart.xml.
The admin template is just a zip of the vmadmin directory.
A bit bigger problem is the AIO updater. Just use the libraries, modules, plugins folder. Rename the Backend module folder to modulesBE, find the installer files in administrator\components\com_virtuemart_allinone and copy them also to the root folder, zip it.
You can also use this tutorial for the plugins https://docs.virtuemart.net/tutorials/installation-migration-upgrade/226-update-single-plugin-delivered-by-aio.html
Or must simple, just buy a membership with complete updated package https://extensions.virtuemart.net/support-updates/virtuemart-membership