They might be similar but not the same at all. PHPportals used vbulletin for it's complete backend only allowing phpnuke modules to be used.
What the vast majority of people, including myself, want is something that will tie the two user tables together, but no other integration.
I use postnuke myself, and they are comming out with a user system that will be the pants off of what either vB or nuke currently using. Hopefully somebody will tie into that, if not then