Vagrant起動時にSSH libraryのエラーがでて、Vagrantが起動できない時があります。
An error occurred in the underlying SSH library that Vagrant uses. The error message is shown below. In many cases, errors from this library are caused by ssh-agent issues. Try disabling your SSH agent or removing some keys and try again. If the problem persists, please report a bug to the net-ssh project. timeout during server version negotiating
この場合、Vagrantfileの以下のコードをコメントアウトすることで、起動できるようになります。
vb.customize ['modifyvm', :id, '--natdnshostresolver1', 'on']
参考URL Vagrant up takes long time after installing guest additions manually in centos box #6812
