Many people have asked me, when I tell them how there is a very small percentage of water on the planet Vulcan, how the Vulcans have survived. Originally my answer was the sheer technology and ingenuity of the Vulcan people. This explanation was not entirely satisfactorily either to me or to those who I gave it to. It was not until I stumbled across some information on Vulcan physiology (while looking for information on Vulcan mating rituals) ((might come in handy if I ever meet a Vulcan girl)).
The Vulcans, as is blatantly Darwinian, have evolved to meet the needs of their environment in some incredible ways. First, their blood is copper based which allows the retention of moisture more than the retention of oxygen. To compensate for this, the Vulcans possess lungs which are fifty percent larger than that of humans. This allows all oxygen and moisture to be utilized before exhalation.
The Vulcans also possess skin which locks moisture in, but which also absorbs moisture into the body. They do not perspirate but use their lungs to exhale heat. This saves on loss of moisture in sweat.
Hope that this gives you a better idea.