@ncw Ah it's almost the same issue as the old cache implementation had. You made this discovery in 2019 and there was even a GitHub issue that was closed upon transitioning to VFS. And this comment (second part). I guess VFS and B2 are just not friends at this time. 
Will try to create some dirs or check into other options.
For now the first thing I did was backup the entire cache dir (1.8TB of stuff I currently don't have elsewhere) into a second ssd. Glad I just installed this new ssd.