Change compile configuration of package installation in brew
brew install vim installs vim 7.4.052
vim --version reports version 7.3


From above observation it is clearly apparent that these are two different vims. By executing vim --version you are invoking vim supplied as part of OS X (you can check this by which vim, which is going to report /usr/bin/vim, while brew-built programs are usually located under /usr/local directory.

To edit vim formula you just execute brew edit vim, yet you do not need to do this just to get your desired farsi support. As you can see from Vim Feature Table, farsi support is included with any build compiled with --with-features=big or huge. As brew builds are compiled with --with-features=huge, they already contain farsi support.


  /usr/local/bin/vim --version
  VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Jan  1 2014 14:31:50)
  MacOS X (unix) version
  Included patches: 1-52
  Compiled by Homebrew
  Huge version without GUI.  Features included (+) or not (-):
  ⋮
  +acl      +farsi      +mouse_netterm  +syntax
  ⋮


To use brew-built version of vim you might like to alias it with alias vim='/usr/local/bin/vim', or try any of other options.