The Vex could be our end, especially since they're practically infinite in number and we still know next to nothing about them. (All of the below is speculation, but with the Vex that's all we can do) However, they can be beaten. We witnessed on the way to No Time To Explain that the Vex had a future of becoming taken before we stopped it. The futures seen in the vault of glass card are only [i]possible[/i] futures, dependent on what we do now in this timeline. The Vex have control over their own time, but they cannot effect our timelines unless we exist in their space (like the vault of glass). This means that there are plenty of ways and opportunities to get in the way of their plans and come out on top. The question is how we do that, and the Vex are probably well aware of their own weaknesses and possible defeats, likely having been defeated before, which means that in order to win we have to exist in a timeline that beats the Vex in a way they haven't experienced before, which becomes harder and harder then more timelines they experience. We as guardians can't be simulated, so the only way for them to know for sure how to stop us from beating them is if they've already been defeated by us before. Eventually, in some timeline, they'll win, but the goal is to secure the timeline we exist in. Now, aside from all the time paradox stuff, the Vex don't even belong in our world. The traveler's influence led them here (on accident, of course) because they hadn't even existed in our solar system before the Traveler, yet they have structures on planets that are said to have existed in time immeasurable. Maybe we could force them back into their own world, too, thus securing our own. We don't understand the Vex, but once we do, I doubt they'll be any more of a threat than the other races are.