I've read some lore on the vex and from what I understand, Atheon can yield results without the necessary cause or it can do things with absolutely no repercussions. Atheon, and the Vex, are using the Vault of Glass to manipulate basic causality so that regardless of what happens or what humanity or any other race does, the Vex will always get what they want. In one of the grimoire cards it states that “multiple causal pathways converge on Atheon” meaning whatever happens on these causal pathways, it's Atheon that decides what happens next, no matter what. Apparently Atheon created the Mythoclast (which translates to destroyer of myths) so that humans can destroy the black heart and or crota, as they are genuine threats to the vex's plans as a whole. What all of this means is that the reason the vex didn't go back in time to kill us is because Atheon planned for us to defeat him to aquire the Mythoclast, so we could destroy the black heart and or crota, so in the end, the Vex win, just like I stated above. Now if we're just talking a full out fight to defeat one another, Atheon and the Vex could easily stop Goku from ever being born if they wanted to. If anything they would just use Goku to get what they want, and in the end, win anyways. Something else I found interesting was the name of one of the weapons you can possibly get to drop in the vault of glass called the vision of confluence. In mathematics, a confluent system is an equation that can be rewritten in many different ways, but still yields the same results.

The guardians are using the travelers light. Also, we [i]are[/i] dead

U know dragonballs?^^