The volumetric grain can usually be improved or eliminated by editing the light source and increasing the quality slider in the "Volumetric" tab. It also helps if you increase the anti-alias in the render settings (number of subrays and/or quality).

Note: as with most sliders in Vue, if you slide them with the mouse you get a certain maximum or minimum values but if you edit the numbers manually, you can go beyond these max/min numbers, in case you need.

I never saw any kind of deterioration of Vue with the increase of poly count, except for the sluggish behaviour (and the occasional crash) at extremely high numbers.