My recollection about this from my physiology class is that pulmonary vasoconstriction in the presence of low O2 allows the blood flow to be redirected towards areas of the lungs where better gas exchange may be taking place, so it may lead to a larger total surface area of gas exchange.
CityLights is correct. Pulmonary vasoconstriction reduces blood flow to areas of the lung that aren't well-ventilated and shunts the blood towards areas that are adequately ventilated.
The vasodilation in the systemic circulation would be an attempt to get more blood per unit time to tissues (there would also be an increased cardiac output & decreased MAP as a result of low arterial pO2, I'd imagine). The way I'm thinking of it, it would be sort of similar to the vasodilatory effect of exercise. Since the muscles require much more oxygen, there's vasodilation, increased CO, etc, to meet the oxygen demand of the muscles (and to carry away CO2 of course).
At least, that's how I think of it (though I could be wrong in my reasoning). Hope this helps.