Hi, Motion blur is not supported for animated RC objects.
The implementation of motion blur in V-Ray is very tricky, because the plugin must provide the position of each segment for the currrent and next frame.
I don't see any way to compute that efficiently, except evaluating RC two times, one for each frame (very complex). We have same problem with FP.
Applying any modifier turn off instancing, and V-Ray handles RC as a standard mesh, computing motion blur internally.