[kh99]

The variable needs to be "registered" to the template. Here's an article on that: https://vborg.vbsupport.ru/showthread.php?t=228078

But essentially, since you want to put it in the SHOWTHREAD template, which of course is an existing template, you'd use vB_Template:reRegister(), like this:

vB_Template::preRegister('SHOWTHREAD', array('mrwrb_forum_large' => $mrwrb_forum_large
));

This needs to go in plugin code at some point after the value of $mrwrb_forum_large has been set.