Why can user vagrant do "sudo su -" without entering password?

I am a new student about vagrant. I just found that I can change to root by "sudo su -" without entering a password when inside the box supervised by Vagrant. Curiously I checked the "/etc/sudoers" and found nothing about user vagrant. Can anyone kindly explain this?

Does sudo always ask for password?

If your timestamp_timeout is zero, sudo always prompts for a password. This feature can be enabled only by the superuser, however. Ordinary users can achieve the same behavior with sudo -k, which forces sudo to prompt for a password on your next sudo command.

/etc/sudoers include another file which contains the NOPASSWD line.

vagrant@ubuntu-12:~$ sudo cat /etc/sudoers.d/vagrant 
%vagrant ALL=NOPASSWD:ALL

Based on http://docs.vagrantup.com/v2/boxes/base.html: "Many aspects of Vagrant expect the default SSH user to have passwordless sudo configured. This lets Vagrant configure networks, mount synced folders, install software, and more."

As moodh mentioned,