IMO it's the other way around, AR is a special case of VR. To make an AR system, you need to basically do all the work involved in making a VR system, then somehow integrate it with a video feed of the outside world, then convincingly add digital content to that video feed in a way that complies with the real world's geometry, which you have to extrapolate from the video. Basically it's much easier to accurately place virtual objects in a virtual world than it is to put virtual objects in the real world. You know the geometry of the virtual world but can only make educated guesses about the geometry of the real world.