Poisoner targets citizen N1, citizen clicks vest button twice (vest and unvest), dies N1 from poison.
In this replay, citizen is #6, poisoner #7. Poisoner options: target knows they're poisoned and dies 3 nights later unchecked, doctors can heal poison and target dies two nights later checked.
I have played with my save with only the doctors can heal poison option checked, and the poison kills next night as intended. Dunno if you made some changes to Poisoner recently.