Vascularity is primarily a function of low body fat in that specific area--but everyone stores fat slightly differently. You cannot control where your body stores fat. In other words, there's no special diet or exercise that causes you to lose fat in a specific place. Doing situps does NOT remove fat from your abdomen.

The only thing you can do is continue to exercise, eat lots of small meals rather than 2-3 big meals, and--most importantly--eat fewer calories than you burn, preferrably with a deficit of no more than about 500 calories per day. As your body gets more and more lean those veins will start to show.

The other obvious "cheat" things are to shave your legs and get a tan.

--Steve