Vector displacement map
Traditional grayscale displacement mapping encodes the difference between the high-resolution model and the low-resolution model, using the low-res model's UV coordinates. Vector displacement uses a similar process, except it also uses the high-res model's UV coordinates, creating a correspondence between the two UVs.
This correspondence allows the map to encode not only how far away one vertex is from the nearest vertex on the other mesh, but also to move the vertex across all three axes in space. This allows the map to encode complex undercuts, for example under a mushroom cap or behind an ear.