the problem can be by some things: improper geometry in one or more of your components, improper materials or the resolution and your configuration.
improper geometry - Sometimes vray can't triangulate the surface. Check your geometry with solid ispector, solid solver or manually.
Materials - Look your vray progress window. In the progress you will see which materials load fast, which slow, other don't load. For those that load slow, you can reduct the resolution and dpi of textures (both of diffuse, bump and displacement). For materials that vray freeze trying loading them, you must change them. It's difficult to find which material freeze exacte, because vray show you only treated materials, so you leave loaded materials and try to repair the others.
The resolution can be a problem too. With my Radeon HD6850 and phoenom II x4, I have a problem last two months. There had a windows update probably and I have no updated catalyst, because AMD do not support catalyst any more. So the maximum of resolution of exterior render of complicated models its 1024x768 for me.