Rules about Motion Blur & Vertex Motion Blur:
The kernel Motion blur act as a main switch; if it's off, the MB modes in Object properties will not be used (considered off).
The vertex motion blur' is the effective on/off switch for PhoenixFD volumes, and if it's turned On, it will use the velocity channel of the volume for MB
The 'object motion blur' is not used for PhoenixFD volumes, the timing is only set from kernel setting 'motion blur duration';
for other mesh objects, set as movable proxy, it just allows to specify the MB 'steps', that affect the timing (ie, the object position) for bluring 2 frames.
Regards
Paride