There are two levels of virtual reality available to experience: low level and high level. Both are useful and compelling for art therapy in the following ways: viewing virtual reality art and creating it, respectively.
On the low / entry level end, Google has created Cardboard. As the name implies, the device is a simple piece of cardboard that folds into a viewer. You add basic lenses that come with it then slide your cell phone into the holder to power it.
There are several websites and apps that allow you to view VR artwork that was created with high end VR equipment. This creates a nice entry way into virtual reality art, allowing the viewer to experience the scale, volume, brushstrokes, and animated qualities of VR forms even if they don't yet have direct access to the high end VR art making tools.
The Cardboard is rotationally tracked, meaning you can look around you in a direction but cannot lean or move around. With the more powerful, computer based configurations you additionally have positional tracking which allows you to lean, duck, jump, walk (and create!) within the virtual environment using your full body.
With high level systems you wear a head mounted display (HMD) that makes you feel present and immersed in a virtual environment- as if you have been transported to a different place. There is the ability to look around you with a wide field of view in true stereoscopic 3D. Spacialized audio provides realistic surround sound cues.
Additionally, hand presence controllers allow you to see your hands and interact with objects. This allows you to push, pull, grasp and throw items often with realistic physics. Having realistic hand tracking and intuitive controls is what really unlocks the possibilities of painting, sculpting, and animating in virtual reality with full body, gestural movement.
When choosing a high end VR headset and purchasing a computer to run it there are a number of factors to consider. In order to get the best form factor and value for virtual reality art-making, I recommend the Oculus Rift and an independently build PC to run it.
My journey with digital art-making tools has lead me to see all media as a creative continuum in service of an initial idea / symbolic equivalent on the journey towards formed expression.
2D---Give form to Idea 3D----Idea occupies space Animation----Idea has movement
Then w/ VR you add interactivity, presence, & immersion and the Idea becomes alive
These stages of an Idea can happen w/ a mix of traditional / digital media or they can all happen within VR (and of course can be mixed non linearly ie 3D Printing VR sculpture or a hand drawn image plane for ref in VR)
Getting into virtual reality at this time represents an exciting opportunity to shape the dialog around a new medium and what it's impact will be on a societal level. Please see the above blue highlighted links for blog posts that delve deeper into these preliminary concerns when beginning the VR journey and don't hesitate to reach out if I can be of assistance.