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.vm.boot_mode
Default value: :headless
Specifies the mode the VM is booted into. By default this is “headless” which makes it so that the VM runs headless, or without a visible display. This can also be changed to “gui” which will show a display. “gui” is useful if you are debugging an issue with Vagrant or require a VM with a windowing system that you can use.