FLEX did an article on supplements that do this a while back, unless you are looking for a quick, un-natural solution. I'll look for it tonite and see if I can find it.
I tried AAKG by itself awhile back, and I'd say it does help after about 3-4 weeks. Then it was only when I was putting stress on the muscles, I wasn't any more vascular just sitting there, only while flexing after some time, lifting, or after cardio. Kind of cool, I never saw the branches in the vein that runs through my right bicep until I supplemented with it. But after stopping supplementing with it, they go back to normal.
Also Virgin Organic Coconut Oil made me more vascular after using it consistantly after a month. About 45 minutes after consuming a table spoon of it something just seemed to turn on big time, as far as energy and vascularity.