You'll want to inform the the duchess of the new information about the murdering beast. Head to the Beauclair Palace to meet her. Needless to say, she isn't pleased about the change in strategy to try to reason with the Dettlaff. One of the cards of the victims found in the toy store has a drop of wine on it. The duchess discovers what type of wine that is. Your next destination is a vineyard in Castel Ravello. Follow her and Damien to the location.
Panthers will be attacking a wagon along the way. It's a pretty straightforward battle. They are vulnerable against Beast Oil. Take them out and continue the trip.
Once you arrive, question the owner of the vineyard to find out about the missing wine. Duchess Anna will get you access to the cellar. Follow her there and inspect the book to the left of the entrance. Then, you'll have to find the 1269 Sangreal. Inspect the barrels using witcher sense. The barrels in question are up the stairs on the second floor of the cellar.
Knocking on all the barrels shows that they are all full. Instead, you'll have to taste the barrels to ensure they are full with the 1269 Sangreal. Find the tap and hammer back on the lower level. The second barrel from the right is the contaminated barrel you're looking for. This will trigger a cutscene of the interrogation of the wine maker. He illegally sold a barrel of the wine and arranged to sell another barrel that night. Meet the buyers to spring a trap.
The plan is to trap the wine buyers to find more information of Dettlaff's blackmailer. They attack once they see Geralt. Take them down with the help of Damien's men. Interrogate one of the buyers to learn about The Cintrian. Another quick cutscene completes the quest.