You're right. I forgot you planned to use this for VR. Eh, it will still work. Let's just say that it would probably be better balanced and might, depending on the complexity of the VR game, show some improvements with a CPU that has more physical cores. But even if that's true you have to gauge whether the difference in performance is worth 300 bucks and I don't think I've said anywhere in this thread that it would be, assuming the use of a 7700k.
There is NO doubt that if you move up to the 8700k you WILL see some noticeable improvements in any kind of gaming environment unless you are playing a title that simply does not tend to use CPU resources much. For VR, that's unlikely. VR uses CPU resources much more stringently AND with VR, even more so than with traditional gaming, you REALLY don't want to drop below the refresh rate of the two displays or it's not going to be a pleasant experience. Of course, there are usually some things you can do with settings but VR is MEANT to be visually pleasing and you bought a GTX 1080TI for a reason so you just really want to make sure that there are not any frame rate drops that are due to the CPU.
If you haven't noticed any problems, then likely YOU should be the one to make that call as to whether you need to upgrade or not. I will tell you one thing, VR is going to get a whole lot more complex and demanding before it gets any easier, as is usually the case even with traditional gaming, so what is working now on a few games is no guarantee that the next generation of titles will be as kind.
If the money isn't the biggest question mark, then upgrade. But only upgrade if you're going to go with the 8700k. I don't see that any other upgrade is going to make a remarkable difference.