> # run openvpn
/usr/sbin/openvpn --cd /etc/openvpn --config /etc/openvpn/acevpn-ultimate-udp-faster-1109-US.conf

Yes - this comes very close - but still a problem with tun0

The fix is described here
https://github.com/dnschneid/crouton/issues/2215

Issue the commands
sudo stop shill
sudo start shill BLACKLISTED_DEVICES=tun0
before starting openvpn

and now it works fine.

There are various ways, e.g.
https://techblog.jeppson...-within-crouton-chroot/
to script these extra steps.