Veins are not all they are cracked up to be.
I'm extremely vascular, much to my own dismay.
I can be at 12-15% body fat, and have spider webs in my shoulders. My hands, I think more due in part to playing guitar than any other factor, are display veins no matter what. I can be 35% (don't think I've ever been above 15-18%) and still have veins on top of my hands.
When I lean out, as I'm currently just about finished doing, people consistently and constantly make comments about them. They are disgusted by them. I can't help it, unless I want to be a lard ass like Goodrum, but I like being lean so I live with the veins.
Like the others have said, it's genetics. Having thin (or thinner than other people) skin, low body fat, and also how you train (to some extent) that all plays part in what your vascular system looks like from the outside.