I would assume it's impossible, and I don't really have a problem with that. I mean, once you hit vengeful, that's the lowest of the low. You have a reputation among the entire faction. You're a known enemy of the state, kill on sight. There isn't any coming back from that, no matter what you do, they will never consider you a friend.
Maybe later there could be a unique mission chain for a couple factions which would allow you to recover.