Reworked the larger stones, generally better result. ZBrush crashed a few times, when I tried to apply some noise. However I found a helpful video on YT that told me to rename an obscure file in an obscure directory, and all was well. How do people ever discover such things.
Actually the stones are not all that great. The roundness and generally rough surface is not in keeping with the very regular cut shapes. Lots of practice needed to get stones looking good. At least the sculpting and normal map process is working well.
So to recap – model and UV unwrap a low poly mesh in Blender, export as obj. Import into ZBrush, maybe make some basic shape changes (move tool), export again. Subdivide to a million polys or so, sculpt, export as obj. Use xNormal to bake a normal map from high to low poly version. Import the low poly version into Vue and apply normal map to the material.