I wonder why you're running a clean site like vimcasts.org on a full-blown CMS. Imho this scenario longs for simpler solutions. Static site generators - in the Ruby universe: toto, nanoc, webby - come to mind.
Well, I guess your passenger/apache is not configured right to deliver the generated static files without using ruby or passenger besides your MaxInstances/PoolSize options.
I remember some issues with passengers and page caching (on nginx e.g.) which needed some tweaks.
I appreciate this burst of traffic to help me get to the bottom of this.