VirtualBox does not work after upgrading to Big Sur
Big Sur user here (11.1), running VirtualBox 6.1.16 and Vagrant 2.2.14. Currently have a VM running, no issues. The major hassles for me came a few OS versions earlier (Catalina), presumably due to tightened macOS security, so may be why it was smooth for me to update to 11.1.
Per the mentioned thread, did you try this?

go to system preferences &gt; security and privacy
unlock to make a change
unblock Oracle (might need a restart)
Install Virtualbox again
Linux VM started properly