I've just completed a move from phorum 2.1.x to vbulletin 2.0... Here's a copy of the script I used to move over the existing phorum posts. This makes everyone appear as "guests" that posted on phorum prior to the move.
It also currently cleans out all posts in the vbulletin forum it's importing into, but the script could be easily modified to not do this.
It also doesn't even try to convert the Hostname in phorum to an IP address. In practice, with 80,000+ posts, it just took too dang long to do all those DNS lookups.

The code to do the lookups in is the script, just disabled at the moment.
Anyway, the script is available for anyone to mess around with/use as a starting point.
-- Rob