Depth of Field (DOF) is not something that you want to use in VR as it can give the user simulation sickness. Because of this, it is not a supported VR feature. The reason for this is that DOF comes from a combination of things like the object’s distance from the camera, the lens that is used and the camera settings (Super high-level explanation of this for simplicity sake).
When using VR, you are trying to simulate the way your eyes work and not how two cameras work so a lot of features that you might normally use, like DOF or vignette camera distortion, for example, will not function as these types of effects do not affect our eyes like they affect cameras. However that is not to say that DOF is not something we see with our eyes, we do, it is just very subtle and not as well defined as a camera might. Because of this DOF will naturally occur in VR when the user tries to focus on objects that are too close or too far way which means you can get a very subtle DOF for free in VR.
As a rule of thumb for VR development in general, you never want to force a VR user to do something that you would find super annoying in real life. For example, you should never forcefully move a VR users head to point in a given direction. That would be the real life equivalent of someone standing behind you and forcibly moving your head to point in a direction each time you move your head slightly. Keeping this in mind can help you avoid some common VR pitfalls that everyone falls into. If you have any more questions about this, please let me know.