The rules stated that we couldn't change Voldemort's utility function or turn him good, but his utility function already placed an extremely high value on not having the world destroyed, or losing his immortality. It was quite possible that the solution would have been to convince him that killing Harry would end the world, or that he required Harry in the future in order to save it. The Vow and the parseltongue both were valuable tools in convincing Voldemort of this.