I asked about the same thing a while back, but was told that there was no automatic script that did this - importing users or threads from one VB to another.
The main problem is that there is overlap of userids and threadids on both forums - you can manually try to merge the 2 dbs by adding a number into the userid field so that when you copy information across you don't delete current information.
I would be interested to know how anyone did this. The merger I had with another board was done by getting all members to sign up again at my board - needless to say we lost a few members, but surprisingly most people did sign up.