isc-dhcp-relay6.service fails to start on boot. This unit should start the dhcpv6-relay daemon. The failure is because the referenced interface does not have an IPv6 address. The interfaces do not have IPv6 addresses because prefix delegation has not completed assigning the addresses.
Version: VyOS 1.4-rolling-202211110728
set service dhcpv6-relay listen-interface eth3
set service dhcpv6-relay upstream-interface eth2 address '2600:1900:3e60:c29f::5
The work around I found was to edit the /usr/lib/systemd/system/isc-dhcp-relay6.service to slow down the daemon restart attempts. The distribution service file includes Restart=always but the default retry speed is too fast to allow the IPv6 address assignments to complete.
I added these statements in the sections shown. This and it works. It appears to take about 20 seconds for the addresses to materialize.