The currently included ISC DHCP server that is used for dhcp-relay services in Vyos does not support relaying DHCP requests to remote DHCP servers that are connected via a GRE tunnel.
As an alternative dhcp-helper is a DHCP and BOOTP relay agent. It listens for DHCP and BOOTP broadcasts on directly connected subnets and relays them to DHCP or BOOTP servers elsewhere. It also relays replies from the remote servers back to partially configured hosts. Once hosts are fully configured they can communicate directly with their servers and no longer need the services of a relay. See dhcp-helper
I have tested dhcp-helper by copying the executable from an existing Debian Jessie host and executed it on VyOS 1.3-beta-202102180958 using
/config/user-data/dhcp-helper -s A.A.A.A -s B.B.B.B -e ethX -e ethY
and it does the job, relaying any DHCP request from any client connected directly to any Vyos (vif) interface (except ethX and ethY) to the DHCP servers at A.A.A.A & B.B.B.B
I know many Vyos users have been struggling getting DHCP Relay to work using ISC DHCP in combination with GRE tunnels. The only tunnel type that seems to work is GRE-Bridge, but that is not an option for us and many others. We use DMVPN, which requires NHRP, mGRE and IPSec.
dhcp-helper just works. No further configuration was needed on the DMVPN HUB router and/or remote DHCP servers.