Vascularity is very genetic. Some guys have veins and others don't. Look at Jay Cutler. No matter how lean he gets, he is never vascular. The thinner your skin gets, the better chance you have of showing off vascularity. So your number one tool for getting freaky veins, is your diet plan. Also, I've found that the longer I train and the more mature my muscle gets, the more veins I have. My veins have gotten wider to deliver more blood to my muscle. So in time, you'll have a better chance of developing more and thicker veins.
As for drugs, really all steroids will increase vascularity, as far as I've seen. The DHT based drugs, specifically Masteron and even more so, Winstrol, are good for vascularity. Also EQ will do a great job bringing out veins, even when not dieting down.