When uninstalling the old version and installing the new version of vagrant (along with the requires system restarts), I was met by this message when trying to boot up my machines.
Bundler, the underlying system used to manage Vagrant plugins, is reporting that a plugin or its dependency can't be found. This is usually caused by manual tampering with the 'plugins.json' file in the Vagrant home directory. To fix this error, please remove that file and reinstall all your plugins using `vagrant plugin install`.
Somewhat confused, I tried running the uninstall/reinstall of Vagrant again, but nothing changed.
Turns out that I just needed to remove the .vagrant.d folder from my Windows User directory.
For me this was: C:\Users\Mike\.vagrant.d
Then I installed Vagrant one last time. After a reboot, my virtual machines booted up again and I am back in business.