No, dehydration does not cause your hand veins to bulge. In fact, it causes the exact opposite. When you are dehydrated, your veins will become much less prominent and almost disappear.
Dehydration means that your body has either lost or used more bodily fluids than it has taken in, and it does not have enough fluid to perform functions as normal. Because of this, blood circulation is not working as it should and the vessels of the vein become smaller as the amount of blood flowing through the veins decreases.