Some folks are blessed with high vascularity. If you are not one of them, then leaness will help.
Cooling is a key....if you ever notice in the summer, if you are under the blanket, when you wake in the morning you will be pretty vascular. This is the bodies way of trying to cool, pumping blood to the warm areas.
If you notice, if you are cold and your body temp lowers your visible veins will disappear until you warm up.
Proper hydration is also essential.
If you are going to compete, there is a topical product called Vazkulizer by dream tan. You may try it. Do a small patch of the body first and see if it bothers you.
Besides that, a blood flow inducer like L Arginine or NO might help. Arginine is the basis of all most of these.