Having trouble getting the latest Vagrant and VirtualBox working together because the ssh key is not acceptable to Vagrant?  Well it turns out that the ownership on authorized_keys is not 600 so it fails in operating systems like Centos.  The trick is to modify one of the files to add a chmod 0600 after the authorized_keys file has been updated.  Full information in the link:

https://github.com/mitchellh/vagrant/issues/7610