To be honest, Baggins, this stuff is way above my head, but my layman way of imagining how it works makes me think that Vector Displacement might be working like a raytracer, but using different vector calculations instead.  When I look at what it does, especially the overshoots, it must be technically impossible for a light ray to come from above, pass an overshoot, and then still have a means of controlling what is below the overshoot.  In other words, it feels to me like it would be impossible for it to control whatever lies in the 'shadow' of the overshoot.

But it does, so ...

It might be drawing a virtual object around the main object, like a cube for example, and then firing off those vectors you speak of, basically just firing them off your object like reflections, and bouncing them off, say, the inside of an invisible cube, so that when they bounce, they are able to reach the undercuts that Standard Displacement can't reach.  Like shining a torch down a narrow maze, the Standard Displacement would fail due to the overshoots, but Vector Displacement, if it works sort of like a raytracer, might be bouncing vectors around like in a raytracer - just done different.