Obtenha o vim com suporte a rubis. Compile-se ou obtenha um binário.
$ wget https://github.com/b4winckler/macvim/tarball/v7.3-53
$ tar xzf v7.3-53
$ cd b4winckler-macvim-730064c/
$ ./configure --enable-rubyinterp
$ make
$ make install (or copy things manually to where you want...)