vbulletin with portal

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hi i have signed up for a portal it is at
http://phpnuke.galacnet.com/oh-twadi/index.php

how do i make it so the log in box logs ppl into my vbulletin website?
what things do i need to have to make this portal work with vbulletin?
i would like ti so that when someone joins the portal it joins the forum and like wise
my forum is at www.oh-twadi.com/forum
thanx