[j883376]

You read the code. Honestly, that's what I've seen a lot of other people say to do. It's personally how I learned how to program for vBulletin. You need to see what's defined in the PHP file for whatever you're trying to do. For example, if you want to do something right before a post is accepted and parsed into the database, you look for the proper hook (They have pretty easy to understand names) and then put in your code in that hook. The proper classes and functions that you'll need will be there elsewhere in the file.

The API is handy because it tells you the arguments for the functions and such, but it's not going to tell you what's in each of the files.