why i failed to login to coreos system which is running inside vagrant?

[mnz hz]

the problem is specified in details with this link:

http://stackoverflow.com/questions/30123262/what-is-the-default-password-of-coreos

(this problem has been posted to coreos user mailling list, but as the document https://coreos.com/docs/running-coreos/platforms/vagrant/ said, this kind of question should be better here,  so it's here now :)