There have been many threads requesting vB's user data be joined with this or that product. I am not offering to do it (ey143 will be after me if I release another hack before his!), but in general, any time you want to link to different tables so that the data is live, you just have to add a routine to send new data back and forth from one to the other and call it any time an update is performed.

For example, to make the vB user database the default for vB and PHPNuke, redirect PHPNuke's user sign up to vB's routine and include some code to update PHPNuke's database after the user is registered in vB's. Do this for editing the Profile, as well. You can leave all the other user stuff alone so long as changes are always initiated in the vB user table and propagated to PHPNuke's user table. The routine just has to map the fields and add defaults for or calculate stuff that vB leaves out (and you do not want to add to vB's table -- hack away!).