For me, the "killer app", or more specifically three basic things to make me use a Vision, would be what I ultimately wanted out of the Hololens when it was released, and was thinking might take three or more iterations to achieve:
Social acceptability. This is something I wasn't counting on Microsoft to get right, and back then in 2016 I even speculated that it would probably take Apple ripping off Hololens, making it idiot friendly and convincing their bovine customer base on it to get there - and here we are on track, if a bit later than I thought. I think this is very likely to happen with Vision gen 3 or maybe even gen 2 if there's a massive leap forwards. This is not just about price of entry, but also obviously about the way the device appears on your head. Right now it's like any other VR headset you wouldn't be caught dead with outside.
A remote desktop. i.e. an AR dual/triple screens at arm's length of of at least WQHD perceived resolution each, along with a gesture keyboard (and I felt again back then a representation of a standard keyboard would be a complete waste of time, even though it looks familiar) and mouse. This means for one that I basically need screens at home for only the most high-resolution needs and be able to interact with any of my desktops anywhere and can even just hide most of my computers away somewhere, also work securely remotely without carrying a laptop... and more of course.
And last but obviously, actually replace the smartphone by having it pack a cell radio in there as well.