Getting proper veins in textures and to a lesser extent bump maps has always been a personal challenge. There are very little references to go on for the vascular system near the surface of the skin. (If anyone has seen a decent medical book detailing visible veins then Im all ears) My notion has been to have multi-texture effects outside of poser where the skin has varying layers of transparency. Starting with a textured oily surface, then a layer for pigmentation like freckles, then vascularity. Depending on the brightness of the lights the more visible the deeper layers. Using Specular layers with a multi channel texture map set up as the lower layer. This would help give the effect of visible freckles in full sunlight. My personal results arent quite what Ive hoped for, but I heard a similar technique was used in the Shrek production.