From Bulbapedia, the community-driven Pokémon encyclopedia.
(Japanese ビネガータウン Vinegar Town) is an anime-exclusive location in Kalos. It is located between Cyllage City and Geosenge Town. It is famous for its chocolate cake, "gâteau au chocolat".
When Ash and his friends came to Vinegar Town, they saw an advertisement for an exhibition match between the Champion of the Kalos region Diantha, and a champion Trainer from Vinegar Town, Morgan. Ash decided he wanted to battle Diantha, and so the group went to Vinegar Town's stadium where the exhibition match would be held. There, they met up with Professor Sycamore who wanted to study Diantha's Gardevoir and its ability to Mega Evolve. Professor Sycamore then introduced them to Diantha.
Afterwards they watched Diantha's exhibition match. Even though her opponent Morgan was described as a champion Trainer, Diantha had no trouble with defeating him. After the match the group wanted to try Vinegar Town's famous gâteau au chocolat. They found out there were long waiting lines in front of the restaurants that were selling it however. Serena was able to get one slice, and the group decided to share it. Right at that moment they met up with Diantha again who was in disguise to also try to get a slice of gâteau au chocolat, but she found out it was sold out completely. The group then decided to share it with Diantha as well.