I believe that directory is created when you have jailshell enabled and a shell user logs in. I imagine it stands for virtual file system.
That's about the extent of my knowledge. I played with jailshell once and with one other test shell user, and it created a file system for that user.... but generally speaking, nobody on my server but me gets shell. I just left the directory there for now (even though the test user no longer exists).
My gut tells me that this file system may be some kind of hard links, so I've been a little leary to delete them. If you have a lot of files in there, I suspect you have shell users, so you likely wouldn't want to delete them anywise.
Maybe somebody with more knowledge than me can shed more light