[kh99]

You need to create a plugin for your php code, create a new php file, or else edit one of the vbulletin files. It's preferable to use a plugin if possible since if you edit any vbulletin files you will need to make the changes again when you upgrade.

You can read about how to create a plugin in the manual, but you also need to find a hook location that lets you do what you want. You can do that by looking at the vbulletin code to fine the calls to fetch_hook(), or you can ask here and someone might be able to tell you.