Every guy who goes to the gym always sees two types of vascular people. One is the skinny guy who has a mass of veins naturally throughout their arms, and the jacked guy who trains hard enough for them to pop out. Now for the every day guy, veins and vascularity rarely show. So I asked some professionals how to become vascular, and they recommended to doing VERY high reps with low weights because this Very high reps of low weights increase capillarity. If this is within your goals, you can incorporate one or two days of high reps low weight into your routine, and within a month or two you will see an increase in veins through the body (mostly arms).
Pre-Workout supplements containing NO2 I believe it is, or nitric oxide, these supplements increase blood flow by dilating the blood vessles in your body. Along with that they increase your energy levels. So if you take this and hit the gym, you will see an increase in size in the muscles known as the "Muscle Pump" and will also see an increase in vascularity as your veins should be much more visible. Although the muscle pump is only temporary, your veins being visable will slowly become a more permanent thing as you work out more, do the high reps routine, and take the supplements to enhance the visionary effects of being vascular.