I compared the logs.
They are identical, except for a few lines:
Mon Jan 9 18:13:20 2017 daemon.notice netifd: Interface ‘VPN_client’ is enabled
Mon Jan 9 18:13:20 2017 daemon.notice netifd: Network device ‘tun0’ link is up
Mon Jan 9 18:13:20 2017 daemon.notice netifd: Interface ‘VPN_client’ has link connectivity
Mon Jan 9 18:13:20 2017 daemon.notice netifd: Interface ‘VPN_client’ is setting up now
Mon Jan 9 18:13:20 2017 daemon.notice netifd: Interface ‘VPN_client’ is now up
Mon Jan 9 18:13:20 2017 user.notice firewall: Reloading firewall due to ifup of VPN_client (tun0)

These lines appear only in the case of start-up vpn from the web-interface.
Any idea how to do it with a script?