There are a bunch of us over in that definitely prefer headsets for this kind of thing. It's a pretty big market as far as I can tell (and personally, I think modern VR stuff is incredible. But to each their own)
Anyway, that thread also has information about how to inject metadata into the file which will let you use your mouse to pan around inside of something like VLC. That's better, typically, than just trying to flatten it, because sometimes you wind up looking down or whatever in actual VR, so just flattening to one portion of the screen is a bad move, and trying to get the whole video to a flat view will look distorted as fuck.