It tends to be very separated, and possibly over-architected in places. At one point, we were using it for session handling (in the DB), and it ended up requiring (IIRC) 10+ classes to do it, on top of the number of DB related classes we had already included.
But it does have some nice components (eg, Zend_Http_Client).
Or in case you are going to use the Kill-list feature to update the index it would be helpful to have a "last_updated" column in the post and thread table with the dateline of the most recent edit. And a log of all deleted posts and threads.
I guess you already know all this. It's just what I've recently been hacking into vBulletin