When upgrading my computer, my openconnect vpn broke. I finally narrowed it down to upgrading network-manager from the version in jessie to the version in stretch. It is network-manager that is causing the trouble. When network-manager only is upgraded, when I connect to my openconnect vpn, the newer broken version sets the mtu on vpn0 to 1500. The previous version that worked, set the mtu to 1406.
