Hi thanks for everyone who replied.... it's not just vascularity and being able to see veins or have a lot of them pronounced. Their Circumference needs to be larger. Like the example I gave of the famous legendary 18 year old pro bodybuilder Casey Viator who Joe Weider, God rest his Soul wrote in his book: "...garden hose veins"....
At plasma donation centers they stick a Super long and thick needle to draw plasma. If you're skinny and you can see your veins like a crack addict, those veins will not withstand a long and thick needle. They basically need to be half dollar coin wide veins, not nickle coin wide veins, for example. I think a combo of heavy weights, low reps, then alternating with low weight high reps, a good diet of low fat foods, and maybe Masternon later on for a long period....and then barbell reverse curls and hammer curls would help...