When rendering nParticles, you must make sure that your cached values in the particle cache are exactly at the time moments that V-Ray will use to sample the geometry for motion blur. You can see which those moments are in the Maya output window when rendering. The problem is that with the nParticles cache, if V-Ray asks for the particle position/velocity at a time moment that is not in the cache, Maya will just return the position/velocity at the closest cache keyframe, rather than interpolate smoothly in time. This is not a problem for the cache of the regular Maya particles where this works correctly. If we were to fix this, we would have to parse the nParticle cache file ourselves to find the time moments of the keyframes, but we decided this was too complicated to implement right now (and should not be the renderer’s job really).
Best regards,
Vlado