The legacy hook used to be in "includes/functions.php" as you probably already know". VBulletin5 uses an API to hook to the core code. Unfortunately, the API is still under development so there is not a concrete guide.
You might want to take a look at this guide:
http://www.vbulletin.com/forum/blogs/joe-d/3932614-
I found reviewing the API source code with the guide above was the most help.
Sorry, I could not be of more assistance.