https://www.virtualbox.org/wiki/Download_Old_Builds_6_1
Here are the steps I took when downgrading. Note that I am not very well-versed in virtual machines and have no idea whether all of these steps are actually necessary, but are just what I did while trying to be very careful of not leaving any messes on my machine.
- Destroyed my virtual machine
- Deleted all the NICs that were created by VirtualBox while trying to bring the machine up
- Uninstalled VirtualBox through the uninstall tool provided in the installer archive
- Restarted my host machine
- Installed 6.1.26 from here: Download_Old_Builds_6_1 – Oracle VM VirtualBox
- Granted the security permission
- Restarted my host machine
- Successfully ran vagrant up