The issue with complete scripts like that is they are designed from the ground up to be standalone. With no forward planning to intergrate into a forum application.
I have looked at similar intergrations beyond user access, but it becomes extremely complex as you start looking at using vbulletin options, vbulletin's template engine, even vbulletins database class.
My personal opinion would be to start from stratch if you're looking for similar functionality, vBulletin can already handle alot of the stuff it seems to be doing, and with a few (probably lots) plugins into certain areas you could achieve it.
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