Completely uninstall VirtualBox and Vagrant and reinstall through brew

virtualbox-vagrant-nuclear-option.sh

# update brew because `brew update` is broken after updating to El Capitan
cd `brew --prefix`
git fetch origin
git reset --hard origin/master
sudo shutdown -r now # restart the computer


# open terminal and run the following
brew update
brew cleanup

# remove all the packages with brew
brew cask rm virtualbox --force
brew cask rm vagrant --force
brew cask rm vagrant-manager --force

# remove any existing packages that might have been missed
rm -rf /Applications/Vagrant* /Applications/VirtualBox
sudo rm -f /usr/bin/vagrant /usr/local/bin/vagrant
sudo rm -rf /opt/vagrant /opt/homebrew-cask/Caskroom/va* /opt/homebrew-cask/Caskroom/virtualbox
rm -rf ~/.vagrant* ~/.docker ~/Virtual*
sudo pkgutil --forget com.vagrant.vagrant

# reinstall virtualbox vagrant and vagrant-manager
brew cleanup
brew update --rebase
brew upgrade --cleanup
brew install vagrant-completion --force
brew cask install virtualbox --force
brew cask install vagrant --force
brew cask install vagrant-manager --force
brew upgrade --cleanup
brew cleanup
brew doctor
sudo shutdown -r now
echo ''