Figured it out. Nothing wrong about the dhcp-server configuration - just a problem with my test methotology. I’m in the process of transfering the 1.1.7 config from the currently active firewall to a new box with VyOS 1.2.3 and my minimal test-environment with firewall + Laptop was of course way too minimalistic.
Not a huge surprise it doesn’t work as expected - with only one link active during dhcp-service start.
Added a switch on each network and now it’s working just fine.
Sorry for wasting your time.