vagrant.d outside of the home folder

By default Vagrant uses ~/.vagrant.d. Fortunately, vagrant provides an environment variable called VAGRANT_HOME by which you can set vagrant home.

Just do the following to change the vagrant home (this only works in the current session)

export VAGRANT_HOME=/path/to/vagrant

To make it permanent, add this to your ~/.bash_profile (for login shell).

Update: VAGRANT_HOME has been added to the documentation - Environmental Variables

VAGRANT_HOME can be set to change the directory where Vagrant stores global state. By default, this is set to ~/.vagrant.d. The Vagrant home directory is where things such as boxes are stored, so it can actually become quite large on disk.

It might be useful to permanently set this on a Windows box by executing

setx VAGRANT_HOME "/d/.vagrant.d/"

On Windows change line 17 of environment.rb located at:
vagrant\embedded\gems\gems\vagrant-1.x.x.dev\lib\vagrant\environment.rb

VAGRANT_HOME does not work well in my Windows 8.1. It worked for me when I changed the file

D:\HashiCorp\Vagrant\embedded\gems\gems\vagrant-1.5.3\lib\vagrant\environment.rb

on line 117 to

@home_path = Util::Platform.fs_real_path("D:/vagrant/home/")

like Steve H said and it works fine.