VPN99 is the VPN interface of the ZS. You will use it when you connect to the ZS over VPN as a gateway to the internal network.
I am not familiar with ZS requirements but your setup seems to be correct. However it looks like NAT on ZS is not working. Try to add a route on the modem for 192.168.1.0/24 via 192.168.0.10 to see then if the modem will reply on pings from inside zone.