Probably this should be an umbrella ticket, but the idea will be create an application to deployment the vCenter Plugin in a standalone way.
In the application you will be able to specific to target the vCenter and point out to deploy the configurations and roles according with the TrueNAS configuration.
Regarding to the download, following conversation with Kris and Support team:
1) host the plugin on www ixsystems.com somewhere for systems connected to internet
2) have instructions so if air-gapped, they can download plugin from that URL, scp it to TrueNAS box into http:// path and then run XML config to fetch locally from TN