Thanks. I knew it had to be something dumb by me.
However...
Here is the plugin now:
//#### Begin Plugin ####
ob_start();
// Included file is located one directory from the Forums, and in another directory
include("../AnotherDirectory/IncludeThisFile.php");
$includephp = ob_get_contents();
ob_end_clean();
// The 'header' template is where I want to output the files
vB_Template:

reRegister('header', array('includephp ' => $includephp));
//#### End Plugin ####
And I tried:
//#### Begin Plugin ####
$includephp = <p>Hello</p>;
vB_Template:

reRegister('header', array('includephp ' => $includephp));
//#### End Plugin ####
And I still get no output either way.
Am I supposed to register the variable in the hook itself, or do I alter the actual code file?
Samwise Galenorn