In the Books of Sorrow, Crota cuts a hole in space/time and the Vex enter Oryx's Throne World (this is a long time before our game's timeline). Starting [url=http://www.ishtar-collective.net/cards/xxxviii-the-partition-of-death]here[/url], the story of Vex in a throne world goes on for a few cards.

Maybe it's a bit of both: [quote]My son,” he said, “this is your punishment. Come home glorious, or die forgotten!” He picked up Crota by the legs and threw him into the Vex gate network. Crota battled through history, becoming a legendary demon. In his early centuries he often spared a few victims to hear oaths and protests against his father. Later, he came to understand Oryx, and he made temples and monuments wherever he went. [/quote] Could the vault have been the holdfast Quiora created to prevent the closure of the way into oryx' throne world? When Crota got cast through it he battled the vex and created a shrine to oryx on the moon?

I'm not sure, I'd have to do some re-reading and think about it to piece it all together, but I believe the timeline doesn't rule it out.

It would mean his finding and fighting humanity to kill the traveler ended up being purely coincidence as he fought the vex through history

Well just off the top of my head I don't think it's coincidence. I believe he was sent to deal with the Traveler as the "four-star general" of the Army, while Oryx, the "commander in chief" called the shots from somewhere else. From the Grimoire you referenced: [quote]Later, he came to understand Oryx, and he made temples and monuments wherever he went.[/quote]

Right. That's what I was referring to when I said Crota is sent to deal with humanity because he screwed up with the vex. Unfortunately we don't have a good sense of timeline for these things. I got the impression that him being cast out of the throne world is what caused him to end up in our solar system.