There is a great thread investigating the same topic that is active on another forum, but since I can't link it, to summarize the ultimate arrived at solution: It comes down to hydration, bodyfat levels, GENETICS, and of course a small and select assortment of tried and true products that will volumize and promote intracellular water retention and enhance NOS levels.
In descending order of importance:
-DNA (Can't supersede that which mom and dad bestowed upon you, which is why even world renowned World Champion Olympia athlete such as Jay Cutler have cycled continually every known anabolic and supplemental aid for DECADES, and gotten into single digit bodyfat countless times and STILL don't exhibit extremely evident vascular tissue)
-Body Fat Levels (While preparing for many 'a contests, I have notice my delts display the most overy vascularity and striations along with my brachial vein, before anything else comes to the surface as I lose body fat. Veins can't push out through adipose tissue)
-Hydration (When you are depleted, you are flat - no matter what. This is the most often encountered plaguing phenomenon that haunts comeptitive bodybuilders on stage after months of preparation and looking like a champion days before the show)
-Nutrition (Carbohydrates... they do a body, and glycogen stores, good)
-Supplements (Last on the list for, obvious, reason. Veg. Glycerine, NTBM BRIDGE, NTBM Need 2 Slin, GPLC, Agmatine Sulfate, and L-Norvaline are choice selections, as of course Arginine and other impostor NOS up-regulator wannabees have been redundantly uncovered clinically, to do nothing)
Although I firmly believe in and have witnessed supplements' ability to enhance the effects of vascularity to a small degree, I'm always reminded of the contest prep I went through earlier this year, using NO/Zero/Nada supplements. I relied, solely, on cardio and nutrition, as my catalyst of recompositioning. Within weeks my delts became feathered and my waist shrunk to to the point of needing to use my training partner's size-small Schiek.
It still comes down to my enumerated principle list of contributing factors: DNA, Body Fat, Nutrition, Hydration.................... Supplements.