I don't have any experience modifying vbulletin, hence why I didn't want to learn what register.php was doing, but you certainly make a good point. It would be annoying to have to fix this whenever the register.php file is updated. I was kind of using
this old thread to help me find where I needed to edit. (apparently I failed to read the last post though)
Thanks for the tip. I am definitely going to move my code to the plugin system and use it for any modifications I do in the future.