Vagrant has a whitelist of approved versions of VirtualBox it can work with hard-coded into it. And that list only gets updated at some point after each new version of VirtualBox has been released. So with each new version of VB there is a window of time where you can completely break Vagrant on your system by upgrading to the new VirtualBox, just because even the most current available version of Vagrant will have a whitelist that does not include the new VirtualBox.
AAAGGGHHH WHYYYYYYY