I'm fixed wing but have touched the perifery of NVGs. Modifications are necessary to cockpit lighting - normal cockpit lighting would blind the pilot, because the NVG is effectively an image intensifier, which amplifies the available light. And it's not just a case of turning the brightness down. All in all it is an expensive mod.
The 2 or 3 public service helio units I have come into contact with (police and ambulance) have been day/night VFR ops only, although the VFR limits have been modified to much lower levels (by special authorisation?)