Well, problem being, the Swarms have no interest in diplomacy or treaties , the only thing that interests them is the total annihilation of galactic civilization. However, you probably won't be dealing with a full hivefleet, most likely just a small portion. And don't worry, I'm not gonna do that, destroy you and all that. I'm not playing to win.

The best that can be done is containment, contain the Vazra to a single world or system and slog it out till you finally wipe out the forces there.