Mac OS X: how to open vim in terminal when double click on a file

Create an Automator Application to run the following applescript:

on run {input}
   set the_path to POSIX path of input
   set cmd to "vim " & quoted form of the_path
   tell application "System Events" to set terminalIsRunning to exists application process "Terminal"
   tell application "Terminal"
      activate
      if terminalIsRunning is true then
         do script with command cmd
      else
         do script with command cmd in window 1
      end if
   end tell
end run

Save the automator application. (eg. name it Vim Launcher)

Right click (or control-click) on your custom vim-type file (eg. use .vim as the extension) and under Open With… choose the bottom option Other… and find your Automator Application (eg. Vim Launcher), double-click it.

Boom.