I have used vascularity maps with the Firefly renderer for years without any problems.  About I year ago I switched to Superfly and started seeing the same weird effects that hborre described: veins that appear to recede from rather than stand out from the surface.  I lodged a support ticket but got a reply along the lines of "well, Superfly is different so what do you expect?"   I thought it might have been fixed with Poser 13 but it's still with us.  

Here's my two cents worth for the Poser brain trust to consider:

I used the standard Poser Surface root node with an old figure and maps, NOT the Cycles node. This makes it easier to compare Superfly and Firefly images. The displacement value has been set to1.25 mm and the subdivision level for the figure is set to 1. The first image shows the figure rendered using Firefly.

The vascularity is not overly prominent: it's most obvious in the groin area and on his elbow.  The next image is the same figure rendered using Superfly with exactly the same Poser root node. I used the Optix GPU render engine, but I get the same results using my computer's CPU, so it's not an NVIDIA thing.  

Woah!  First, the displacement is unrealistically large.  It certainly isn't 1.25 mm! (Raising the subdivision level doesn't seem to change anything).  The second thing is that some veins stand out while others recede.  The scene uses 2 infinite lights. Rotating the figure does seem to change where the receding veins are located, an effect that hborre has also mentioned. 

The final render is the same as the previous one but with the displacement set to 0.125 mm.  

The veins now look much more realistic, but there are still receding veins (especially in the figure's right arm).  

All this suggests to me that there is a problem in the way Superfly handles displacement.  The "receding" effect may be a fault in the way shadows are calculated, and the magnitude of the displacement is unphysical (a displacement of 0.125 mm should be essentially invisible).  

Displacement maps are also used for non-human figures and props (walls, etc.) so this problem has wider implications.