]I did it! With help from vBulletin-germany I could find the line where you need to add the code.
Simply add it to the html and php keywords in functions_wysiwyg.php.
So replace:
Code:
'#\[(html|php)\]((?>[^\[]+?|(?R)|.))*\[/\\1\]#siUe', // strip html from php tags
to (whatever your tagsname is, here I added highlight)
Code:
'#\[(html|php|highlight)\]((?>[^\[]+?|(?R)|.))*\[/\\1\]#siUe', // strip html from php tags