this are some compatibilty issues with your local installed grunt and this module!
if you have grunt 0.3.x installed (what i think is your problem) you could use an older version of this plugin (should be grunt-vows@0.3.1 or older).
probably the best thing you can do is upgrade your local installed grunt to 0.4.0 (see the migration guide here: http://gruntjs.com/upgrading-from-0.3-to-0.4), because you will run into this issue a lot if you want to include new grunt-plugins.
of course this will add some additional work in already existing gruntfiles