Oh god, something drastic, I think. It would be hard, though, unless there was something to bind him to his word? Because otherwise he'd just promise and then SHOOM out of there with Boyd when she did the whatever it was thing. But I could see him promising to like, I don't know, frame Daniel for yet another murder to tighten her hold on him or something. I mean he'd literally do ANYTHING. Even have sex with her. Gasp. He might point out that if the mafia discovered Vaughn could have saved her and didn't, it would be her ass in the fryer. Dude, I don't know. It really would depend on the context. Epic idea, though.
I mean I wonder if "I will hunt you to the ends of the earth and tear you apart piece by piece" would be enough. Because if Boyd died under those circumstances HOLY SHIT would Shane go Terminator.