the site has more than 1.2MB of locally-stored images on the homepage... if the site has a daily visitorship >10K, and depending on user cache settings, that's many gigs of overhead per day. on a low bandwidth, memory limited VPS, apache serving up that much content could be a real factor. I'm not speaking from experience, I'm speaking in general terms having done a ton of research on VPS' at one point and deciding to stick with shared hosting until I outgrew it.
Maybe mark (or others) can speak to the issues of huge content like that; someone with lighttpd or alternate serving approaches would be helpful. Also be good if Will could provide us with the VPS specs, so we know exactly what the min and burst CPU, RAM, and pipe are, and what processes are running on the box aside from LAMP.
I noticed the homepage is now showing it is cached, with a dynamic-serve time of 0.320 seconds. Given other feedback above, seems like things weren't getting and staying cached...