The dhcpv6 server configuration is not exported when none of the prefixes are present with the following error:

None of the DHCPv6 subnets are connected to a subnet6 on this machine.
At least one subnet6 must be connected such that DHCPv6 listens on an interface!

When all IPv6 prefixes are delegated they are not available at the time the configuration is generated causing this failure. If the configuration is loaded and then re-committed after delegation when the configuration succeeds.

This check is performed here:
https://github.com/vyos/vyos-1x/blob/current/src/conf_mode/service_dhcpv6-server.py#L208

When this check is commented out, the configuration exports and in my testing by the time kea-dhcp6 starts the prefixes have been delegated and the dhcpv6 service performs as expected.