David ;-)I'm as you with Trisquel 7 mini but Xfce, nevertheless Network Manager is identical whatever DE you are under.
You have to click on Network Manager applet, then modify your current ethernet connection settings.
Click on Network Mnager applet > Modify (etho) > In the General Settings choose : Connect Automatically to this VPN, you will be prompted with your root password, save & exit, reboot if you wish.
