You're viewing the documentation for the experimental version of Vagrant (the 1.x series). The current stable series is 1.0.x and the documentation can be found here. You can learn more about the Vagrant versioning scheme here.
Configuration key: config.ssh.username
Default value: "vagrant"
This is the username that Vagrant will use when attempting any sort of SSH-based communication with the VM. Most public base boxes are setup in a way such that the “vagrant” user is used.