Have you asked the guys over at vbulletin.org which is the vbulletin hacking/mod site where
vb users talk with each other about mod ideas?
You can not really ask a mod question to vbulletin.com which is the company that releases vbulletin as code modification is not a supportable action so your only recourse is the site I noted above that I know of. There may be other sites out there you can talk about modifying vbulletin but I know not know them off hand.
Sorry but I have not used
vb 3 in many years and like the vbulletin company that releases vbulletin we are mainly here to support our products as they are released we do not discuss code mods either.
I can tell you generally that vb3 is pretty easy to work with from what I remember it is a simple add php code and echo results kind of environment.
The only thing that gets loaded in the headerinclude template of vbulletin would be any type of support javascript functions. You would then load anything else in the script or templates for where you want the function to work out. That is a general outlay of how things work I would think feel free to discuss whatever mod your doing to
vb over at vbulletin.org