That number has the potential to be quite rubbery, depending on how they collect the data. I can see a situation where I might be listed as a VR player, I have the hardware, but I have only played Elite in VR for a couple of hours, as impressive as it looks, it's not for me. VR FPS is more my thing. So I wonder how that would mess with the numbers, because I'm likely not the only person with VR gear that doesn't play ED in VR.
Quite honestly, I doubt they know.
While FD are very good with metrics, it's all stuff they have on their servers. Playing ED in VR is simply a graphics setting on the local client (selecting an HMD option), and I don't see how or why they would know about that.
I can think of a bunch of ways to capture that info. It could be something as simple as a flag on your login, or it could be something more rigorous like querying if a VR API is running.. Either way, there are a number of options.
Look at it this way, Open, PG and Solo are simply options on the client, but the game knows exactly how to deal with the selected option, why would VR be any different?