The successfully downloaded Vagrant boxes are located at ~/.vagrant.d/boxes on Mac/Linux System and %USERPROFILE%/.vagrant.d/boxes on Windows. This can be changed by setting an environment variable named VAGRANT_HOME to specify the location of .vagrant.d, as in VAGRANT_HOME=F:\.vagrant.d

# Windows
# current terminal session
set VAGRANT_HOME=F:\.vagrant.d
# current user environment
setx VAGRANT_HOME F:\.vagrant.d
# system wide environment
setx VAGRANT_HOME F:\.vagrant.d /M

VAGRANT_HOME is documented here along with other interesting options.