The Vault of Glass has two purposes: 1. To write the Vex into reality itself as laws of the universe, which would allow the Vex to manifest the powers of the Templar, Oracles, etc. outside the Vault. 2. To mimic the power of a Hive throne world, and attempt to copy the Hive's logic and power by victory. Hence the Glass Throne

I knew 2, and I'm pretty sure the VoG was either teleported there or crashed from Mercury. Prob not true :)

It most likely appeared when the Traveler terraformed Venus