I’m sorry alzhao, but it doesn’t work.
When I push the button “Save & Apply” in /etc/config/firewall I find these lines of text

config forwarding
	option dest 'lan'
	option src 'VPN_client'

config forwarding
	option dest 'VPN_client'
	option src 'lan'

but when I reboot, I find this lines

config forwarding 'forwarding_vpn1'
option dest 'VPN_client'
option src 'lan'

Why?