I have always noticed more vascularity with of course a lower body fat percentage, but there's nothing like some high rep, constant tension sets to get the blood pumping like crazy. Here's an example of an exercise that kicks the **** outta the veins in my bi's. Grab some dumbells that are a little lighter than your normal standing dumbell supinating curl, hold one at a 90 degree angle (90 degrees between upper arm and forearm) while you curl with the other. The burn is insane especially if you go for 5 each arm then try for say 3 more each arm without ever setting the weights down, but push through it and watch the veins swell!