Hello Sigbjørn
Since there is no VehicleDiscovery service, to get the valid Vehicle IDs you ned to get them form the provider (Operator/Authority...). More generally, SIRI requires some reference data (Network IDs, Stop IDs, Line IDs, Vehicle IDs, etc.) that you need in order to be able to build you requests (VM, ET, SM, etc.)
Cordialement
Christophe