Marshalus: that bug has to do with database switching...some versions of PHP/mySQL do not handle it well. It works fine for me, and I can't reproduce that error, though people seem to have it.
pdatoon: Importing into a board with users/posts already there works just as well as when you're importing into a blank board. There is only one concern: say a user registered on your phpBB as 'Nebula'. Then, you shut the phpBB down, move to vBulletin for a few weeks. Nebula creates a new account on the vBulletin. If he names that account 'Nebula', then my script will link all the old phpBB posts to his new username. However, if he registered as 'NebulaDJ', then my script would create a new user 'Nebula', instead of 'NebulaDJ'. The official importer has a step called 'Associate Usernames', where you specify a username on the phpBB, and it can link it to the current vBulletin one. All my script does is check if the usernames are the same. So, if this is important to you, use the official one. If this doesn't matter, you can use mine or the official one...