Pantunes,
That is a standard default route out of the device to the internet and not related at all to the VPN configuration. If that route were missing the VPN tunnel would not be up at all as the gateway would not be reachable.
Route based vpn need a route to the tunnel interface as part of the VPN configuration.
set routing-options static route 192.168.168.0/24 next-hop st0.0
This is not required for Policy based VPN.
Yes, I think all the testing done here shows the next place to look is the VPN configuration on the remote side for the Zyxel firewall. From what I see in searches some models require route and policy configurations to use tunnels and others have a fully integrated wizard setup. Which model is in use for this connection?