With default firewall rules, (INPUT and FORWARD to ACCEPT), you will be able to reach everything, if you aren’t able, I’d investigate on the rules, eg. input from VPN99 must be allowed, as well as the forward from VPN99 to … (what you need/prefer)…
Also, why I cannot ping my VPN’s IP address of the ZeroShell? Just to explain.
I used default configuration where IP address of VPN99 is 192.168.250.254. DHCP of the VPN is 192.168.250.1-192.168.250.253. Default gateway and DNS are 192.168.250.254.
When I login using VPN client, I received 192.168.250.1 IP address. I cannot ping anything, including 192.168.250.254 address. Message is Destination Host Unreachable.