When you render cached nParticles with V-Ray, motion blur will not always render. This is because, to get the motion blur data, V-Ray reads the cache file at specific keyframes. If these keyframes do not match the frames that motion blur was originally recorded on in the nParticles animation, then motion blur cannot be rendered. However, we can amend this discrepancy with the Use Interpolated Cache option from the Extra V-Ray Particle Attributes. Follow the steps to learn more.
Keep in mind that this is only the case for cached nParticles, if they are created in the current scene, this workflow does not apply.