It's not possible for the impostor to win in a game that's down to the final 2, impostor vs Vampire. If the impostor kills the Vampire, they will lose, despite being the sole surviving killer, because the logic does not check to ensure that a Vampire is still alive in that situation.
Because of this line of code: https://github.com/eDonnes124/Town-Of-Us-R/blob/eddc89eda75251541e0e9b82d0c9b40df6ea8aff/source/Patches/Roles/Vampire.cs#L41
It's not possible for the impostor to win in a game that's down to the final 2, impostor vs Vampire. If the impostor kills the Vampire, they will lose, despite being the sole surviving killer, because the logic does not check to ensure that a Vampire is still alive in that situation.