Sweet! A chance for me to flex my VR nerd muscles! *cracks shoulders and neck* Okay here we go~
A video game on the same, grand scale as the ones shown in SAO isn't impossible to pull off. Imagine a game with the kind of immersive Sound Design that GTAV has, (PBS Game/Show has a video about GTA's sound design on youtube) along with realistic lighting & environment textures...combine that with attractive anime style character models that have a soft cel-shaded look, along with all of the world building, monsters, quests, GUI, etc; and you pretty much have an SAO experience. It's not something that's easy to make, but with the right talent on board, it's a do-able thing.
Making that in a VRMMORPG is a little trickier. For some people, VR automatically means motion sickness, and while that is steadily being phased out, it crops back up again if you're in a massively multiplayer online game. Even high end computers experience lag in MMO's sometimes, so imagine you're in VR and you're fighting a monster and it suddenly jump-cuts you to another spot during the fight. I imagine that would be pretty disorienting.
Unless the VRMMO has simpler graphics like LoE does. I've heard that some people have already tried VR with LoE before, and from what I've heard, it works pretty darn well
So there might be hope with that!