That's a real good question.
I was wondering the same thing - so I bought one.
So far, when I pop my mi 5s into the USB-C plug on the mi VR2 (white plastic) , it immediately pops up some full-screen text-on-black background warning (only in chinese) with a 1-2-3 step of removing the phone from the goggles.
it then pops up some overlay (also only in chinese) with buttons 'OK' or some other symbol. Clicking them does nothing.
latest CHINESE OFFICIAL and BETA roms. These automatically download a different version of mi VR app that's available around various filehosting websites (have yet to find an xiaomi official host....) when you plug in your headset.
This all works rather seamlessly to be honest.... if you speak chinese. There is no english option.
What DOESNT work:
V 8.0.3 MAGMIDI (MIUI8) (MIUI GLOBAL OFFICIAL) (MI VR NOT SUPPORTED)
V7.3.9 v8-6.0.zip ( MIUI EU WEEKLY UNOFFICIAL) (MI VR NOT SUPPORTED)
The official product website references a "3. MIUI VR edition: will be released together with MIUI 8. " -- not sure what or where this is.
For now, I have a rather clunky and odd-looking paperweight.