I recommend the vBulletin files themselves.
If you are beyond the scope of index.php, pick open global.php, functions.php and if your feeling l33t, init.php

You will find, as I did, some nice coding that will help you in your coding career. Want the best way of learning? Convert a (large) hack from another forum software.
With the above exersises, you will learn new ways to improve your coding as you go along, and as a bonus: There's no better way of learning vB coding