As others have mentioned, it's LOW BODYFAT. Seriously, do you think you will be vascular all over legs and arms at 16 or 18% bodyfat? And that's probably the typical realistic bf of all the gym rats that work out year after year at my gym "bulking up" forever but never cutting down.
Do the 10-12 weeks of hard work, drop down to 10% or lower, and if you are on something as simple as 500mg of test, it will be pretty obvious as the veins pop out everywhere. It's not as sexy as "use tren 75mg EOD, plus 100mg Anadrol and then throw in some NOxplode pre-workout", but it works.
One warning: every time I got to what I considered great vascularity, my hemocrit levels were above the safe level. Could be just me, but just something to keep in mind for long-term health.