I have seen different vB boards on different servers sharing a particular forum, they call it "the network forum", and all the threads and posts in that forum are shared from board to board.
Here is an example:
http://www.theranter.com/forum/vBull...splay.php?f=51
That forum and all of its threads / contents are shared with another vb board on a different server, with this vB board:
http://www.anothermessageboard.com/f...splay.php?f=29 (you must register to view this one though)