Interestingly, the amount of CNX2 vignette adjustment is very dependent on the image. For example, if I fully reverse vignette an f/4 image taken with the 16-35 the corners are very dark. if I do the same to an f/11 image, there is very little adjustment at either extreme end of the slider.
CNX2 may somehow evaluate the image, or it may rely on a table of values based on lens, focal length and aperture. The logic would be a secret sauce that we could only speculate about, although forcing an unnatural vignette by stacking filters or something similar might shed some light on it.
I think it is obvious though, that it is far more than a "dumb" radial ND grad type of adjustment, such as the tool that Capture provides.
But regardless of the logic, the only information that the raw file can pass to the raw rendering image is the value of the vignette option - off, low, medium or high. It is up to the app to take that simple instruction and do something useful with it. And for that reason, a 3rd party app like aperture could never fully emulate what CNX2 does. At best it could read the metadata tag and do it's own interpretation of that very crude instruction.
It would be very possible for Nikon to build in a table of lens/focal length/aperture settings of its modern chipped lenses because Nikon knows all those details about its lenses. I don't think it would be reasonable to ever expect or hope that a 3rd party app could gather all that data about all the lenses in the world (made by many other makers such as Canon, Sony, Sigma, etc).