Anecdotal evidence suggests that patenting activity in the field of virtual reality has sharply increased in the recent past.
I would not use the US classification system for patent analytics anymore, as it is essentially obsolete and is being replaced by the Cooperative Patent Classification (CPC). All published patent applications and patents also have International Patent Classification (IPC) codes. Codes specific to virtual reality were recently introduced to reflect the increased activity in the area.
Here are some classification codes often used for VR-related documents:
IPC: G06F3/01, G06F3/0354
CPC: G06F3/011, G06F3/0346
3D graphics in general:
CPC: G06T19/00 and also specifically G06T19/006 (mixed reality)
Games with virtual camera coupled to tracked HMD:
CPC / IPC: A63F13/5255, often in combination with A63F13/211
For more information see here: