Is systemic veins considered capacitance vessels?

Yes, systemic veins are considered capacitance vessels.

Capacitance vessels are blood vessels that can store a large volume of blood and thus play an important role in regulating blood pressure. The systemic veins, which include the vena cava and its tributaries, have thin walls and a large lumen, which allows them to store a significant amount of blood. When blood volume increases, such as during exercise or after a meal, the systemic veins can accommodate the extra blood and help to maintain blood pressure within a normal range.