The vault of glass is just that a vault. There's nothing in the lore that even hints to it being a different dimension, considering the way vex portals work those portals only take us through time. It different dimensions. The vault of glass is a vex structure buried deep beneath Venus that houses atheon times conflux (he basically controls all vex portals) and he is protected by the Templar and gorgons. It's not another dimension but rather a very complex vex structure/base.