This is how I got Vim to do syntax highlighting for JSON files (Max OS X - Snow Leopard)
- Download the syntax file from http://www.vim.org/scripts/script.php?script_id=1945. (I got json.vim version 0.4)
 - If it doesn't already exist, create a .vim/ftplugin in your home directory.
 - Put json.vim into .vim/ftplugin
 - Add the following lines in your .vimrc file (mine is located at ~/.vimrc)
 
au BufRead,BufNewFile *.json set filetype=json  
au! Syntax json source /Users/brad/.vim/ftplugin/json.vim
Note that you'll have to change the path to your json.vim file so that it works on your computer. 5. Now, got edit a .json file!