Well, Vagrant has to find the Vagrantfile to read it, doesn't it? =)
So you either have to be in the same directory or any subdirectory below it. Or you can set VAGRANT_CWD
environment variable to point to the directory. See the "Lookup Path" section in the Vagrantfile documentation for more information.
You can of course make wrapper script or other shortcuts if you need to use that often.
Btw, you might want to upgrade your Vagrantfiles to use V2 configuration format to use all new features of Vagrant 1.1+.