We actually don't know where the Vex ruins came from. Ghost says they predate Humanity by a billion years or something (I'll have to double check for confirmation, but other sources suggest that when the Traveler terraformed our Solar System, it pulled parts of the planet's from other timeline, thus pulling the Vex into our universe. But we actually don't really know where and when the Vex came from.
It's unknown when and how the Fallen made first contact with Humanity. Given the desperate state of both species, it's quite easy to imagine that either one could have fired off the first shot.
Regardless of who did it, the Fallen began attacking us and we've been at war ever since. The closest we've gotten to diplomacy with the Fallen is with Variks, the Loyal.
This is only a theory. But with Destiny 2 starting on Mars. And knowing all Cabal leader ship has been killed. We can probably suspect a major invasion. Maybe a bigger fleet then Oryxs. Remembering the rejected concept art of the city burning I can guess the city could be destroyed in Destiny 2. After the city the Cabal probably wouldn't stop there. Putting the House of Devils and House of Kings in trouble. Maybe sparking a possible alliance? You know more lore then I do so does this sound possible. I would love to be able to become a fallen