Vagrant logs in to the guest as the "vagrant" user and uses sudo
configuration to not require a password for admin privileged commands.
An ubuntu box sets a random password for the root user. If you
absolutely have to use the root user to login, you should set a
password for the root user.
--
# After logging in as vagrant
$ sudo passwd -u root
--
You should probably also check that the ssh config [/etc/ssh/
sshd_config] hasn't disabled remote root ssh sessions.
An alternate is to add your ssh PUBLIC key to the root user's
authorized_keys file. (you /could/ use the vagrant users public key if
this machine isn't holding critical or sensitive data. Not recommended
as the private key is in the public domain)
HTH
jasherai