I apologize if I'm not understanding your question correctly but in Corona you currently cannot have the camera "inside" the VDB cloud - it won't render properly if you do. Same goes for volumetric effects that aren't global. So if you have a box with a Volume MTL applied to it you won't be able to position the camera inside that box and get proper volumetrics going.
We are aware of this limitation and we're keeping track of the requests so thank you for getting in touch!
When the camera is inside the VBD, the cloud is not rendered. After the camera is pulled out of the VDB frame, the cloud can be rendered. How to set it to be rendered all the time? thanks