[spoiler]this is my first attempt at a lore post...[/spoiler] So everybody that has played destiny essentially knows the vex are capable of time travel, but then why haven't they gone back to the past to kill us? It's because they didn't exist in that time period... The vex came from a portal that Crota created in his ascendant realm, the vex then started their rapid transgressions upon the solar system once they were able to exist in this dimension. You may now be wondering, how does this matter? It's because the vex only travel through time, not through other dimensions. Because of this the vex cannot travel to a point where they did not even exist yet. But then how do they teleport to places? Well they don't teleport in the traditional sense, they skip through time to a place that they have been or will be at in another timeline. The vex teleports that create the "fog" are from another timeline where the vex were already there. Then how do the portals work? Well they include the timeline "jumps" but they work like a wireless network where once they go into a portal they can be instantly teleported to another portal, like an email between two computers. TL:DR The vex didn't kill us in the past because they did not exist in the past, the vex jump through time not space, and the portals work like emails between two computers.