I belive the vex classifications are like a tree. Starting with a broad group and superheating into smaller units. But I do belive that if given time,(and that's something the vex have a lot of) could very well destroy the city. But the vex collective is not a war machine. Their primary function is construction and terraforming. They only attack when they see something as a threat to the building. That's the reason they attack the cabal. The cabal like to test vex areas and take control of them. Guardians don't have the numbers to accomplish this. This somewhat relates to the vault. Why, of all places, put it next to the light? Because we aren't a threat. The vex new of the taken threat and use us as antibodies to fight off the taken threat. When the taken entered the vault, they released the signal and we went into the vault and defeated the threat. We are no where near a threat to them.