You have to make a profile to detect you're home (e.g. connected to home Wifi, or based on location).
Then execute a Send Intent action. Use the following entries with the Send Intent to start the Open VPN connection:
Action: android.intent.action.VIEW
Cat: none
Extra: the name of the OVPN profile you want to use
Package: net.openvpn.openvpn
Class: net.openvpn.openvpn.OpenVPNClient
Target: Activity
You can leave other entries empty.
To stop the connection: same as above, except:
Leave Extra empty
Class: net.openvpn.openvpn.OpenVPNDisconnect
I use it to start OpenVPN automatically when connected to public hotspots I use on a regular base that are not all that secure (or block my secure imap mail).