I've suggested it to vb in their forums but would like to see how difficult it would be to create a hack. Here's the idea:
I want to make it so adding an event would be similar to adding a Poll in a forum. Just like with a Poll you'd see an "Event" designation next to the thread title in the the forumdisplay view with a representative icon. When you open the thread you'd see the event details up at the top just like you would see the Poll information. I would even go as far as to add functionality that allows people to RSVP just as if they were voting in a Poll. The event would then be automatically posted in a corresponding calendar.
In the admincp the admin would define which forums could have events added to them and you'd be able to designate a calendar ID to be associated with each forum.
The poll functionality is already there. I would think that code could be used as a starting point to do this.
If you'd like to add to this suggestion on vb.com so the vb developers can hear your voice you'll find the thread here:
http://www.vbulletin.com/forum/showthread.php?t=257348