Yes. The UCI for “force_vpn” is in this file /etc/config/glconfig

You can modify the UCI value then execute /usr/bin/setvpnfirewall

Do you know how to modify the switch button script?

Actually you don’t need to modify the UCI value. The script does all these thing:

To use normal Internet:

setvpnfirewall --disable

To always use VPN

setvpnfirewall --force