arch在狹義上是拱門，約等於壹個二維的結構。舉個例子，凱旋門的英文是triumphal arch，由此可以知道這裏arch用來形容的是“門”，壹個二維的結構。可以同時參考壹下英文釋義：a curved symmetrical structure spanning an opening and typically supporting the weight of a bridge, roof, or wall above it. ? an arched structure forming a passageway or a ceremonial monument: a triumphal arch. ? a shape resembling an arch or a thing with such a shape: the delicate arch of his eyebrows. ? the inner side of the foot.
vault表示三維的拱頂，是多個arch在不同二維平面組成的三維結構。英文釋義見：1 a roof in the form of an arch or a series of arches, typical of churches and other large, formal buildings. ? literary a thing resembling an arched roof, especially the sky: the vault of heaven. ? Anatomy the arched roof of a cavity, especially that of the skull: the cranial vault.
個人覺得這二者的關系近似中國古建築中的椽、檁、梁、枋與整個屋頂的關系。