i'd like to build a writing cache layer for my zfs pool, is vfs-mode write/full helpful in this case?
Could you elaborate a bit more what exactly you want to do?
ZFS has a very robust read/write caching system on its own. I do not understand what your VFS layer would be for.
Said that - rclone is open source project with MIT license. It means you can use/reuse any parts of it for whatever you wish easily.
zfs has very good read cache, but no write cache, not at all.
I suggest you do some google and reading e.g.:
ZFS is fantastic file system and if you are one of this people who contribute (or plan) to its development I salute you.
Like I said rclone is released under MIT license so you can use any part of it in your quest to improve ZFS.
And think where your VFS layer will cache data... In the same ZFS filesystem you are trying to improve?
I suggest you read the article carefully before you suggest it to others.
That's also why an SLOG isn't truly a cache, it is a log like its name suggests.
Now stop playing around, please, you are not helping.