[eluser]Shadab[/eluser]
I was trying to achieve the same thing.
But found out that vBulletin can't be initialized from inside a function.

[ Reference: http://www.vbulletin.org/forum/showthread.php?t=205765 ]

Now trying another approach:
» Initialize vBulletin and pass the $vbulletin object to a (CI) library somehow.

Will report tomorrow.