I've added a var_file from the command line using a variable inside of the extra-vars option on the command line. Something like...
$ ansible-playbook all some_plabook.yml --extra-vars="myvarfile=/path/to/varfile"
Where some_plabook.yml has a:
vars_files: {{ myvarfile }}
It's a bit verbose and ugly as opposed to having an explicit var-file option, but it works.
<tim/>