It's not an exploit with vBUlletin, no one is hacking into the forums.. The host (Dreamhost) is shutting down connections with the server when it reaches a max cpu / memory load with the server, alot of shared hosting accounts do this, Dreamhost is somewhat worst then others when it comes to this. This site has grown too much for it to be on any kind of shared server.
Mike, have you checked out Webhosting Talk? (
http://www.webhostingtalk.com)