That's simply not true - you can't actually detect grain at 3200 samples per inch. What you're seeing is not grain, but grain aliasing, and this effect is actually reduced by scanning at a higher optical resolution, if your scanner is capable of it.

Velvia (the original version) actually suffers from a phenomenon called "pepper grain", which again isn't true grain - it's actually particles in the film base. This effect can be reduced on my scanner (Minolta Scan Multi Pro) by diffusing the light source.