for those who are looking for a command-line solution, here you go:
first of all, you can list the existing VPN connections you created under this path:
sudo \ls -l /etc/NetworkManager/system-connections/
after finding which VPN connection you want auto-connect, edit the corresponding file (
my-VPN in my case)
sudo \vi /etc/NetworkManager/system-connections/my-VPN
... and change the
autoconnect=true, save and exit;
sudo from the command if you are under root user.
[sudo] nmcli connection edit for interactive command-line connection editor.