Roblox has announced that they have gone VR making them, what appears to be the first MMO to go VR. The official post by them is here: http://blog.roblox.com/2016/04/roblox-enters-the-vr-space-with-launch-on-oculus-rift/ Roblox is a widely known MMO that now has the advantage on any others currently out there.
“The experience of ROBLOX on Oculus Rift is similar to other ROBLOX platforms. It is fully cross-platform, meaning that if you’re on Oculus Rift, your friend is on PC, and another friend is on mobile, you can all jump into the same game server. No other games on Oculus Rift are or any VR tech offer that level of connectivity.” quoted from the link above, they currently only have the Oculus Rift capability, however, in an interview they stated they will be reaching out to all other VR devices, and trying to incorporate them as well. The game has been available for around a month now, and they stated they have most main sites, and almost all features fully VR capable. This shows that pre-existing MMO’s are fully capable of doing this, however, when they will is entirely up to them.