You mention polygon count isn't important. ? Since when. When working with round objects the minum sub division needed for the around object to appear round is 8 -12 sides with normal smoothing. 12 sides is basically the best middle ground. Pending the object of course, with any thing over 24 sides being negligible. Additionally even with VR you could always apply LOD to trees, pretty much all 3D game engines support it to some extend. Most engines also have a tree system in place or plugin tree system that can reduce quality or billboard trees the further it is away from the camera. Implying polygons isn't important is daft, especially with VR were the player is able to view props, level design, and texture detail in much closer detail compared to FPS or 3rd person view, with the last two a lot more forgiving. The last two have capsules for collision purposes prevent cameras from getting up really close to objects. VR still uses a capsule of sorts but the placement of the camera is different to that of FPS and 3rd person. It pretty much the basics here. Whatever is accessible to the player, needs to be highest reasonable detail, what isn't can be reduced or omitted entirely, whatever you add polygon wise somewhere can be taken away somewhere else. Bumping the tree up a bit is barely going to have a significant enough impact to FPS. You can do it with less than 200 - 400 polygons.
So yeah for VR polygon count and texture quality is vitally important.
The tree system in place can render a 100 000 thousand trees without issue, Automatically scale rotate and you can have up to 16 different tree types in a particular biome. The simple trick is that it billboards the trees after a certain distance from the camera.You don't even need a LOD based model as it automatically switches over to billboards. Both unity and unreal can achieve similar effects.
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I personally finished off a project that is releasing on Friday on steam as DLC for an engine based on the wicked engine, which has a realtime dynamic lighting system and has VR support, and has a realtime sky box.
Designing a modular system like this for an engine that doesn't have a primitives, was fun. Polygons particularly because of the round shape of the environment, is vital.