It's very much not a bug, so it's a good thing you didn't post it as one
Before I get down to the instructions, I just want to take a moment and explain why we did this: Back in ye olden days, we required to you fill out a THIS_SCRIPT value field (basically, forum.php is "index", forumdisplay.php is "forumdisplay" etc) for all pages you wanted the shoutbox to appear in.
That worked around this issue, it'd only display for the actual locations you wanted them in.
However, the volume of support requests from people who had no idea how to get it to show up on forumdisplay or even a custom page was getting ridiculous rather quickly. That's not saying anything bad about the users - I don't know how my keyboard actually works (although I am convinced sorcery is at work), so if my keyboard broke I would need to get help from someone who works with hardware.
In other words; it was very user un-friendly.
Thus, we insert the code on every page and give people an easier-to-read description of how to get the SB on a custom page with no more technical knowledge needed. We now only get 2 kinds of support requests; people who want it only for a specific forum ID, and people who are concerned about the page load times / unnecessary code evaluation.
With that out of the way, the plugin in question is "Global Initialisation
#2 ".
You can either edit the plugin in the ACP, and wrap your if()'s around the require() in there, or you can edit /dbtech/vbshout/hooks/process_templates_complete.php
