Vasodilation = HOT.
When your body temperature is hot, blood vessles in the skin dilate, so more blood to flow through to the skin capillaries. This means that more heat is lost from the surface of the skin, through sweating.
Vasoconstriction = COLD.
When your body is cold, blood vessels in the skin constrict, reducing the amount of blood that flows through the skin capillaries. This means that less heat is lost from the surface of the skin by radiation.