Currently VR can have suboptimal performance when using the Vulkan backend (#138977). Jereon has run various experiments and was unable to resolve the issue. So in the meantime work on Vulkan VR performance has been delayed until Blender 5.0. Since Blender 4.5 is likely to release without a fix for the VR performance issue. This commit adds an explicit note about it to the user preferences. Pull Request: https://projects.blender.org/blender/blender/pulls/139519