I’d recommend you use deluge’s ability to download to a temporary folder and move on completion to the mount. It won’t use any extra space compared to writing to vfs cache which is essentially trying to do the same thing.
Also if you’re trying to seed from the mount, be aware, if the mount itself disconnects/isn’t available to deluge for any given reason when it’s seeding actively deluge will mark the file as needing a force re-check, which on a remote mount takes a very long time.