In Xenial 16.04, I was able to use Network Manager to configure a VPN to autostart per network connection.
In Bionic 18.04, I can't yet find this option.
Is it available via some configuration or additional package?
See also: https://askubuntu.com/a/1033606/11052 and Automatically turn on VPN on computer unlock Ubuntu 18.04
While Gnome 3's connection editor doesn't (as of writing) offer the option, it's possible to configure this via nm-connection-editor as before in 16.04
See Automatically turn on VPN on computer unlock Ubuntu 18.04 for instructions - the question talks about unlocking, but the answer applies to other network connection changes too.