Thanks Hannes. There are some threads about vdisp, but it's actually quite easy. In mudbox (for one) you can pull out shapes in a subdivided plane, the add a base lores plane, and have a (world space) vector displacement map extracted (calculates the difference). This is a 32-bit color tif or exr (mine was only 8Mb). Import it using an image map shader, set colors to 'are lineair', set a decent size and attach to a vector displacement shader. I only had to negate the Z for this to work properly (don't know why). Then set size of displacement. Etcetera...
