It would work as OP describes; if you got off the death attack first you would check to see if the defender recognized the betrayal (if yes, no insta kill, but it's still a hidden strike attack), then the defender would roll a save if they hadn't (if fail die or become paralyzed), then damage would be done, and then you would decide what appropriate vigilante talents to apply (so if they failed to identify your sk, but made their death save, you could then choose to apply mighty ambush or a different vigilante talent you know which is marked by an *).
@Rysky; you're thinking of rogue talents, which have to be declared before the attack. Vigilante talents work slightly differently; they instead only activate if the defender is unaware of the attacking stalker, or if they consider them an ally (or the ability text has some exception to those rules), but you don't have to decide which one to use first.
