This should be
vagrant plugin expunge
(I haven’t tried it yet, but getting the right command is a good start.)
It worked - in that the command ran, removed the plugins and reinstalled them.
It didn’t work - in that the same error occurred.
Finally: I found a similar github issue which pointed out that user had two plugins called hostmanager, and that one of them is not related to vagrant.
I, too, had hostmanager and vagrant-hostmanager installed.
vagrant plugin uninstall hostmanager
removed hostmanager and also rubyhacks, which is where the error was.