As Aetherys feeds Vilk another dose of the poison, Vilk's thrashing becomes more violent
Fortitude Save against Lycanthropy:1d20 + 5 ⇒ (14) + 5 = 19 Fortitude Save against Wolfsbane Poison:1d20 + 7 ⇒ (14) + 7 = 21
Vilk reverts to his human form and does not die from Constitution damage.
Yay, you win.
By ending the Midnight Mauler’s reign of terror before the city guard tracks Vilk down, the PCs spare the Pathfinder Society from an awkward situation that may have eventually led to the entire Society’s expulsion from Ustalav, barring heroic deeds or extraordinary blackmail.
If the PCs manage to save his life, Vilk recovers consciousness around sunrise. He thanks them, but otherwise remains silent and withdrawn, ashamed of his actions. Within a week, he leaves Ustalav for Varisia, where he eventually settles into the hustle and bustle of Korvosa to live out his final years. Parties that save Vilk earn the Master of Blades Saved boon on their Chronicle sheets.