If PHP is available, add the tool jf as a JSON Formatter by Composer:
$ composer global require codegear/json-formatter
Then Format current file:
Or add a keymap in vimrc:
nnoremap <Leader>jf :%!jf %<CR>
In a moment of serendipity, I discovered that a comment block in a different file was not reformatted! The only difference was this file used a different set of 'ftplugin' and 'syntax' configuration.
After a LONG bit of detective work I finally traced the solution to changing my comment settings from...
Unfortunately, I can't find anything to force Vim to exclude some text region from the manual formatting. But at least the following script should do this:
" format a line range except the lines starting with a 'comment' string
function! MyFormat(comment) range
" go to the firstline; then mark the lastline
execute a:firstline ';' a:lastline "k'"