If you have to edit json files from vim, you may want to make them more readable, here is how you can do this:
start by installing the JSON::XS perl module from the CPAN
sudo cpan JSON::XS
then, edit your .vimrc and add the following
map <leader>jt <Esc>:%!json_xs -f json -t json-pretty<CR>
now while editing a json file, you can hit ,jt (or whatever your leader is set to) and tidify a json.