I've been experimenting with ways to increase vascularity for several months. Low body fat is a must, but aside from that there are ways to increase vascularity in the gym. Taking a good pump enhancer helps. Also carb depleting for at least four hours prior to your workout (more is better) and then eating a lot of simple carbs pre-workout does wonders. This is an old Rich Gaspari trick. Addtionally, eliminating caffeine as much as possible helps. Caffeine contracts the blood vessels and restricts blood flow. Lastly, using high reps for a really good pump. Using this procedure, I have discovered veins that I never new existed in the past.