I find it easier to draw veins right on the figure with Painter 3D or Deep Paint 3D using a stylus and tablet. If you play around with brush settings you can get a rounded look. The main problem I found was that the texture maps for the different body parts are different sizes. If you draw a vein down the forearm it gets really huge when you get to the hand so you have to adjust your brush for that. I also found the book "An Atlas of Anatomy for Artists" by Fritz Schider very helpful as it has a lot of surface anatomy pictures.