I have php files well any file type in general that I would like vim to highlight here docs sections using a different syntax highlighting.
<?php
echo <<<HTMLDOC
<h1> I am highlighted as html </h1>
<<<HTMLDOC
echo 'High Lighted as PHP again '
?>
You can tweak the colors of course but try this:
:highlight MyHereDoc ctermbg=green guibg=green
:match MyHereDoc /<<<\([A-Za-z]*\)\_.*\1/
Also, if you plan on it being something more perminant in your vimrc add:
highlight MyHereDoc ctermbg=green guibg=green
matchadd("MyHereDoc", '<<<\([A-Za-z]*\)\_.*\1')