This is like actually headache can be solved easly, I was trying to get volume vector AOV and could not get it to work so I need to create custom render passes. I use arnold at the moment. It's the king for volume anyway.
First you need to have simple volume with vel field. this can be achieved with simple gas solver or simple sop vector field.
Rename any field you wanna render. Make sure it doesn't have a conflict with houdini naming term. example I was trying to read vel in volume sample and it wasnt working end up with black image. So for vel I rename it to velocity.
Then read this attribute in volume sample, hook it to emission in ai standard volume. and crack up the emission. Now we can read attribute we wanted easly. For volume motion vector pass so that nuke can read properly you need to transform from world space to camera. Arnold has its node to change this.
That' all guys, This is can speed up yout worlflow since volume render motion blur is expensive at the moment.