The help text for the subnet-id field in the DHCP server configuration states, that the value can be 0, however it fails when setting it to 0.
[edit service dhcp-server shared-network-name NET2 subnet 192.0.2.0/24]
vyos@vyos# set subnet-id
Possible completions:
<0-4294967295> Unique subnet ID
[edit service dhcp-server shared-network-name NET2 subnet 192.0.2.0/24]
vyos@vyos# set subnet-id 0
Number is not in any of allowed ranges
Invalid value
Value validation failed
Set failed
[edit service dhcp-server shared-network-name NET2 subnet 192.0.2.0/24]
vyos@vyos# set subnet-id 1
[edit service dhcp-server shared-network-name NET2 subnet 192.0.2.0/24]
vyos@vyos#