Hopefully this will be everything you needed to know about Virtual Reality in P3D V4 at present. Note, The below is based on my experience on a Oculus Rift. I won't bother listing my PC spec as it makes no difference because VR performance in v4 seems identical to VR in v3.4 (see performance below)   ** New *** Update at Bottom from Lockheed Martin - 2 June 2017. Also reworded a few items that were unclear.   Quick Q & A's
Q) Can you now click on everything in the VR Cockpit view?
A) Yes, you can. As far as I can tell everything is clickible in VR now (however see below) Q) Are there issues with when clicking on the cockpit items in VR?
A) Yes, actually quite a few (see detailed section at bottom) Q) Does any of the popup windows (like ATC, Map etc) work in VR?
A) No, They only pop up on your 2D monitor... bloody annoying. It means you have to take your headset off to interact with any pop up window. Q) Does HDR Lighting work in VR now?
A) Yes it does. The Saturation level is low as default which makes the green, green grass of good old blighty look like the Nairobi desert but once ramped up things looked good. Q) Do all the anti aliasing options work in VR now.
A) Yeah but personally I don't think its anything to get excited about as I find the best visual improvement comes by ramping up the screen resolution via super-sampling. Therefore I can't claim to have tested this to any depth.
 
Q) Can you adjust your view in VR now?
A) Nope. You get the option to reset your view to the default position and thats about it. All the "eyepoint" movement controls don't seem to work in VR. However, you can alter the eyepoint setting in the aircraft config file to change the default view is you wish (However it is a faff).
 The only other VR view setting you can change is turning stereoscopic view off and on (see more about this at the bottom)
To set your default view to VR, you need to do this strange procedure...
Choose your plane and scenario. Once sat in the 2D cockpit, choose the VR cockpit from the view menu. You should now be in VR. Then exit back to the scenario selection screen and save (and overwrite) the scenario. Now each time you choose that Scrnario you will be inVR automatically. Q) Do you still get the annoying spiny clouds when you move your head in VR?
A) Sadly yes, you still get that. Q) Is the performance any better in V4?
A) If it is, it is not by much. It certainly isn't worse. I started getting landscape stutter when I ramped up the settings to roughly the same point I got the same stutter on V3. However the autogen distance is far further now, so in that respect you get more distance in your view for roughly the same performance.  Q) Can you control the VR resolution?
A) Yes, but not in native P3D. The Oculus it runs at whatever supersample settings you set in the oculus tray tool (same wiith ASW). I was running at 4k (Super sample 2.0). I assume it will be the same on the Vive. Q) Can you control the Oculus VR ASW?
A) Yes, but again not in native P3D. (use the Oculus tray tool) Q) Is the image quality any better?
A) Maybe slightly yes (when compared to P3D V3 and Flyinside at same resolution). Not sure why... maybe HDR???? Q) Would you buy this for the VR features?
A) At the moment No. The VR is little different to V3 at present and there is no Flyinside option ATM either. You would probably only buy V4 now due to all the other none VR improvements. However, VR alone only really brings you HDR and not very good mouse controls at present.   The Many Problems with the VR clickable Cockpit There are many issues with the clickable cockpit in its Current form. 1) Two on screen Crosshairs (one for each eye). The left and right eye render a crosshair in slightly different positions. So you see two cross hairs about 8cm apart. The Left eye is the one that is most accurate... but even that has issues  (see point 2 below). Strangely, the higher you look up into the sky they more they merge into one cross hair (which would be frigging useful on the actual cockpit controls!)
2) The right cross hair is useless but even the left crosshair can be off target. I found that different planes had different button positioning accuracy. The Maule was way off with pretty much everything. You have to click way below and to the left of the button you need. (E.g. to press the comms pri/alt switch you actually have to click the comms on/off button). 
3) The Crosshair doesn't change when hovering above a clickable item. Therefor you have no idea if you are in the right place to click
4) Changing the monitor 2D screen from maximised windowed mode to a smaller Window seems to upset the alignment even more. Therefore you need to keep your 2D screen at either maximised Window or full screen view.
5) Popup screens (ATC, MAP etc) can loose your VR crosshair. As you move the crosshair in VR, it also moves the mouse on the 2D monitor. Therefore if you move the mouse over a popup screen (which you can't see in VR) and then click, you loose the Crosshair in VR as you have now swapped to the popup windows.
6) The crosshair moves with your head in VR. Everywhere you move your head the crosshair follows. This might be great if you could assign the mouse clicks to a controller (and use your head to select buttons etc). However, there doesn't seem to be any way you can do this natively in P3D, so its just there like a fly on your nose (unless you move it out your way with the mouse)
7) Turning off Stereoscopic doesn't really help. Stereoscopic is the thing that renders a slightly different view in VR to give you a 3D effect. Turning it off solves the "two crosshair" thing (and improves FPS) as you now have the same view in each eye (but still get the VR head movement). The downsides are that you loose 3D depth and everything looks flatish. The clicking is helped because there is now only one crosshair. However, I don't see this as a solution because for me it does break immersion and it doesn't solve issues 2-6 above (inc the crosshair being off target in the Maule). The only good thing about this feature is you can assign a button to switch stereoscopic on and off on the fly.   The known Future for P3D V4 VR
As usual LM haven't really commented on how soon they will fix these issues (if at all). It is probably low on their things to do list. It has been mentioned that the P3D v4 SDK has some VR improvements but I haven't yet got any understanding how this can bee used to improve the native V4 yet? *** Updated comment from lockheed Martin - 2 June 2017 *** We fixed two more issues. Here is the updated list:
- Stereo and mono performance is the same when mono should be faster. (fixed)
- Performance is degraded after exiting VR. (fixed)
Workaround: Resetting the scenario should return performance back to normal levels.
- Label text mismatch between eyes. (fixed)
Workaround: Run in mono mode.
- Dynamic lighting renders in left eye only. (fixed)
Workaround: Run in mono mode.
- Mouse cursor alignment issues. (Planning to look into this soon)
Partial workaround: Run with app maximized for best alignment.

Issues marked fixed will be included in the next release. At this time, we can't comment on when that will be. Thank you for your feedback and patience. Please keep the feedback and detailed bug reports coming.
Thanks http://www.prepar3d.com/forum/viewtopic.php?f=6317&t=124675&start=15#p158608 ********************************************************************************
Flyinside did a good job of creating a VR add on for P3D v3. They have said that v4 will get their attention at some point but they are stuck bug fixing their X-plane release. Therefore a P3D v4 release is month(s) away.   P3D V4 Vive
I have not used a vive so I will add any comments that people chip in for anything drastically different on the vive