I’m no expert with regards to this, but I’ll share my understanding of the impact of VR.
First off, i’d say that the CPU is critical to framerate irrespective of whether your playing in VR or monitor, and MSFS is CPU intensive in general. However because VR is more sensitive to changes in frame rates (and thus it is more important to maintain consistent frame rates in VR than with a monitor), for this reason I’d say having a good CPU is more critical when in VR.
Now in order to manage the load on the CPU, you can adjust the Object and Terrain LOD settings - as these impact CPU but not GPU. So in the case with your 9600k, you will probably find that you will have to run lower LOD settings in VR to maintain consistent 30fps. However as a 4080 will be mismatched with the 9600k (a lot more powerful), you could find that your CPU is bottlenecking the GPU - unable to provide instructions quickly enough to the GPU. In this case, you may not be able to fully utilize the GPU, as it spends time waiting for instructions from the CPU.
So to answer your question - yes it is likely that the 9600k will bottleneck the 4080, and to adjust for this you will have to lower you LOD even further. Also even if you run with all non LOD graphics settings on high, it is likely that you will experience a mediocre framerate, with your CPU being 100% utlized while your GPU will be loaded much less.
My suggestion is to upgrade the CPU at the same time, or soon after buying a new 4080.