To begin with, I’m sorry for not having any idea on why this might not be possible.
But, it would be convenient to allow both the built-in DHCP server to have listen address(es) and listen interface(s) simultaniously. As of VyOS Stream 2025.11, this is not possible although I have not checked the latest rolling version at the time of posting.
A case where it might be useful is to assign an IPv4 address to a downstream L3 switch while also distributing a default gateway, and then allowing said switch to relay DHCP requests to the set listening address.
Thanks.