Night Vision is simply pure military strategy. It's invaluable to combat situations that take place in dark areas. So it would have been invented in some form. Personally? I'd rather see a type of red-LED flashlight instead of traditional nightvision - because the light a red light gives off doesn't hurt our natural night vision and it still illuminates things. But that's really not the point. From a realistic standpoint, it's common sense that they were invented. They might be bulkier than they are in real life, but they'd likely have some from of existence.
^
Personally, I'd like it if any helmet could be upgraded to have Night Vision capability (this excludes hats; the item would have to, at full repair, give you a higher damage resistance than 1). Let's take, for example, the Combat Helmet and the Power Helmet (I am focusing on these since they will both be in the game, and both will probably be used extensively). With the combat Helmet, this 'upgrade' (you'd probably have to go to a certain person who has the skills to make these 'special goggles', and then have to find the equipment to make them [probably with some electronics parts, and a pair of binoculars]). Once you have all the parts together, he'll ask which helmet you want to put them on (or, hey, maybe it'll be a special helmet that has them equipped, might be easier than making them get 'added on' to the current helmet). If you were to put them on a Combat Helmet, they would look like the clip-on-flip-down binoculars that the U.S. Military currently use in night ops. If added to a power helmet, they would just give you a NVG display on the visor (aka the helmet would have it installed inside itself). Now, if the NVGs were made with their own helmet (which I think would be easier for Black Isle to do), then you'd just put on the helmet (and there might be an activation key, i.e. you press 'Left Bumper' or 'F' or something and they turn on. Alternatively, they might be 'always on' when you put the helmet on).