Integrating vB Login with existing Login System of 3m members

[Dunhamzzz]

I work for a web company who own over 100 websites, we have just
completed our own "passport" login system for our all our sites,
currently we have almost 3 million members.

Our login system is portable to our other sites and runs off a webservice.
We have just released a network of 48 linked sites with our own
custom-made forums but despite being integrated and built around our
passport login system, the forum itself lacks the useful features that
full-fledged software like vB does.

I am pushing for vbulletin to my employers and would mainly like to know
(as it would be me doing the work), how easy would it be to integrate vB
with our existing login system, we have simple php functions like
"isValid()" to check logins and system the returns a callback page so
when people login we can effectively run any sql/set sessions or anything.

Some very abstract code examples would be ace as I need some evidence of the ease of integration to put forward in a meeting.

Thanks in advance.