The hidden shaders Unity uses for Unity terrain grass detail patches falls back in VRChat to shaders that are not affected by fog settings and have cutout rendermode, forcing world builders to use the tree mesh part of terrain to render grass patches and that impacts performance a lot.

The two hidden shaders in question are "Hidden/TerrainEngine/Details/BillboardWavingDoublePass" and "Hidden/TerrainEngine/Details/WavingDoublePass" causing all the grass in a scene to stick out of the fog.

Dropping in this replacement shader that adds alpha transparency doesn't get picked up in VRC and gets reverted to the cutout shader without fog: https://github.com/Nition/UnityTerrainGrass

Screenshots from Unity 2017 and VRC 2017 build showing no alpha transparency and fogging in VRC but working fine in Unity