By default the kea DHCP server does strict client-id matching.
This breaks network boot setups with static ip assignments (as the network boot agent and operating system will have different client ids).
The solution is to set the match-client-id setting to false. This already works when manually editing the config file and restarting the service.
See also https://kea.readthedocs.io/en/latest/arm/dhcp4-srv.html#using-client-identifier-and-hardware-address
Can this setting be exposed in the VyOS configuration?