By default, vim doesn’t recognise the syntaxing options for the /etc/php-fpm.conf files. So if you edit them, there’s no syntax highlighting whatsoever.
If you want that, add the following line to your ~/.vimrc file:
syntax on autocmd BufRead,BufNewFile /etc/php-fpm.conf set syntax=dosini autocmd BufRead,BufNewFile /etc/php-fpm.d/*.conf set syntax=dosini
That enables your syntax highlighting in Vim and sets the file “/etc/php-fpm.conf” and “/etc/php-fpm.d/*.conf” to follow the syntax rules defined in the “dosini” set. Other syntax highlighting options are defined in “/usr/share/vim/vim*/syntax/*.vim”. Each .vim file there can be used as the syntax option in the “syntax=” line in your vimrc file.