[dreamhost]

I can't be of much help with phpnuke as I chose postnuke instead years ago and have never looked at the phpnuke code, though I know it's quite similar. What I can say is there is actually a whole lot more to bridging the applications than you are talking about in this thread.
Things to consider:
1. registration using either vb or nuke has to populate the other db properly.
2. vb uses md5+salt where nuke does not, so you have some cookie issues to deal with.
3. a way for the users to modify their profiles in one place needs to be done so that the other system will recognize the changes to their profile, i.e. password change
4. the administration of accounts such as the admin changing a users pass for them needs to effect both systems.
5. How to display vb within pn is a huge process to overcome. There are really 2 ways that I see of doing it. One is to just use the vb header/footer and designing them to match the nuke theme. The second is to actually turn vb into a true nuke module without touching the vb or nuke code itself through bridge scripting. This is really the toughest part of the entire process but well worth it. It's just my opinion but simply using vb header/footer instead of using it as a module is just plain tacky. Why cause your members to loose their side navigation just to be in the forums......
Anyhow, I hope this post is food for thought in your journey to bridge the applications. To read more check out this thread:
https://vborg.vbsupport.ru/showthread.php?t=65633
We not only managed to unify the registration and login system of both vbulletin and postnuke, but also turned it into a true module without touching the code other than a few code inserts. Being able to upgrade either pn or vb in the future with ease was a huge priority of ours.
I wish you luck.