The downside of building in stuff is that you can't easily replace it. I don't have any VMS experience, but the versioned file systems I've studied are very far from being a replacement for what I would consider version control today. They're more like the automatic writing of backup files in Emacs.

Distributed FS sounds like something with a huge design space. It's better to do that in userspace. Was Plan 9 really the first time virtual file systems could be implemented in userspace? It seems like such an obviously useful idea in retrospect.

The diff3 utility for 3-way merge appeared after the VMS debut (1979).

https://en.wikipedia.org/wiki/Diff3

Can you really call what we see in VMS full revision control when it lacks this capability?