When you enter the command :help syn-file
they say:
Create your user runtime directory. You would normally use the first item
of the 'runtimepath' option. Example for Unix:
mkdir ~/.vim
Create a directory in there called "syntax". For Unix:
mkdir ~/.vim/syntax
Write the Vim syntax file. Or download one from the internet. Then write
it in your syntax directory. For example, for the "mine" syntax:
:w ~/.vim/syntax/mine.vim
Now you can start using your syntax file manually:
:set syntax=mine
You don't have to exit Vim to use this.
I think that should do the trick for your problem.
Also I'm not sure how buggy is your original file but this answer may interest you. For the tl;dr version: including the first line #!/bin/bash
(or whatever is the shell you're using) helps vim to determine which syntax should be used (but I don't know if it is relevant in your case).
~/.vim/syntax
?/usr/share/vim/vim74/syntax
is by "Dr. Chip" too. It's just an older version of the file.