I can see the policy defined under Networking routing Rules, no idea how the VPN is set to use that table I am afraid.
The reason 0.0.0.0/1 sort of works is 0.0.0.0/1 is a longer prefix match than 0.0.0.0/0 (i.e 1 bit longer prefix). So anything from 0.0.0.0 to 127.255.255.255 will prefer the 0.0.0.0/1 over the /0. Router will use the longest match first, then decide by metric.
Simon