I’ve wondered for some time - what is the difference between these two scenarios?
1. An object with Vector Displacement, with only the “Subdivision and Displacement Quality” attribute assigned
2. The same object with both “Subdivision and Displacement Quality” and “Subdivision” attributes assigned
Obviously in scenario 1 subdivision still happens, but the result will be different in scenario 2 (even on a simple plane, which doesn’t actually change topology when subdivided). What exactly is going on “under the hood”?
Hopefully someone else understands the question..! 
When Subdivision attribute is added V-Ray will render the surface as a Maya Subdivision Surface which is not the same thing as V-Ray Displacement subdivision.
Here is an example of Subdivision attribute added to a plane:
The actual mesh is as follows:
The way that Displacement subdivides the geometry is as follows:
Subdivision and Displacement quality is just an override of the Default Displacement values from the Render Settings.
A combination of Displacement and Subdivision attributes will actually mix the above two results and it would look like as follows:
Thx for the reply. So am I correct in saying that the reason we get quite different results with Vector Displacement is that VRay is effectively subdividing our base geo first, before applying the displacement?
