Well we've managed to stump 2 separate "server optimization" companies and vBulletin developers along with misc other server admins who have thrown in their 2 cents with our problems. It's not as if we're just brushing this under the rug, it's that no one has been able to come up with a working solution to the problem.

Next step is to try something somewhat drastic like switching the web server software, which would result in a downtime of a day or so, thus far we've only been updating already installed software. We're really trying to avoid getting more hardware, but that might be the only option ... short of switching to different forum software as I'm sure users would complain to high heaven. vBulletin is one of the most feature packed forums, as such switching to something else would result in the loss of some features.