It might be a good idea, at this stage, to take a reality check and reassess the function of (night) flying aids.
For flying in any type of rule, the aim is to be able to: (a) control the aircraft; and (b) avoid obstacles.
For flight in instrument conditions; there is the added requirement that the helicopter be certificated in accordance with Appendix B of Part 27/29 (this certification is mainly concerned with stability and handling qualities and ensures that the helicopter can be flown by the average pilot, on instruments, without exceptional abilities). Certification for flight by a single pilot (SPIFR) also takes into consideration the necessity to perform tasks other than flying the helicopter. The instrument rules consist of a number of elements one of which is the minimum height above obstacles (a number of acronyms are used to describe this height). The two elements of visually flying - in IFR - occur in the departure and the visual segment of the arrival. The use of EVS (at least in fixed wing) is to improve the acquisition of the required visual references in order to complete the landing following an IFR flight - this improved acquisition can be used to obtain approval for a lower minima.
For flight in visual conditions; control of the aircraft is maintained by reference (only) to external visual cues, and avoidance of obstacles is achieved by flying at a minimum height (mandated by some States) using see and avoid.
Flying VFR at night; requires sufficient visual cues to be able to maintain control using external references. There are no specified (additional) certification elements for VFR flight at night (although a number of us feel that there should be a link between usable visual cue environment and handling qualities as a certification issue). In some States (and in ICAO Annex 2 - Rules of the Air), VFR is predicated upon minimum visibility and distance from cloud and not a visual cueing environment. Some States - like the USA - specify a level of visual light cues for night VFR; some States - like the UK - use terminology to imply that adequate visual cues must exist. The use of NVIS (NVG systems) is intended to: improve the cueing environment so that that control can be achieved by using the enhanced visual cues seen through the goggles; and improve obstacle avoidance (in conjunction with some formal procedural elements).
In summary:
EVS is intended to be used to improve instrument flying efficiency - specifically in the visual approach phase; NVG to enable visual flight at night where there are insufficient visual cues to maintain control (unaided) with external references.
Flying at night under VFR and keeping your head in the cockpit is a recipe for disaster. There is no halfway house; it is VFR or IFR and applying the full set of rules.
Jim