I found some discussions from a few years ago, and it seems that fast sync was not supported for Vulkan at that time. Does the latest driver support it now?
https://www.reddit.com/r/nvidia/comments/fusczu/has_fast_sync_been_stealthily_updated_to_work/
https://nvidia.custhelp.com/app/answers/detail/a_id/4160/related/1