Your scene renders pretty fast here (3m 25s), but i have not VRayEmbree installed (i got the missing plugin warning at load). This plugin is experimental, and if i remember well it does not support instancing. Probably it is creating all the problems in this case. Please, try removing vrayembree2013.dlo from your configuration.
Btw, you should increase the VRay Dynamic memory limit. It's set to 400 Mb., very low if you add complexity to the scene. See the rendering guide for the suggested settings.