I agree that when you get "grain" (sample rate too low in the render engine), you need to increase the "quality" slider(s) and maybe also add more anti-aliasing. Depends on the lighting type, materials (e.g. blurred reflections) and if soft shadows are used. There is more control available for Spectral lighting, for example.

My free tutorial on volumetric lighting has some useful tips - though not about "graininess".
http://www.art-head-start.com/free-stuff-tutes.html

Another option is to use a 3rd party program that removes digital noise - then you can render faster in Vue. However a fix inside Vue itself is best. Neat Image is an excellent 3rd party program used by photographers to remove digital noise.

http://www.neatimage.com/