Mapping Image Maps to Lateral and Normal displacement

[WAS]

I think I am confusing myself more than I need to. I'm trying to figure out the best way to map displacement maps that are both normal and lateral? If they are image maps, how do I map them correctly to the underlying terrain? Get normal seems to capture the displacement the image maps create, which breaks it's data of the underlying terrain.

[Dune]

I think you'd really need vector displacement maps instead of the flat 2D grey displacement maps.