Virgil's Eevee couldn't have evolved into Sylveon since X/Y weren't even announced by the time he appeared. He could appear later so that they could showcase how Eevee evolves into Sylveon, but since the only League rival to appear after a League was Ritchie, it not too likely that Virgil will appear again.Virgil was about as interesting as Kenny.
Except that Kenny got to evolve his Piplup on up, while Virgil didn't even get a Sylveon.
I don't remember the anime showcasing Pokemon that weren't even announced yet, at least not for a long time. After X/Y were announced, I'm pretty sure that the Unova League was already over, so they couldn't have given him a Sylveon. Plus, if they did, they probably would want to show how Sylveon evolved, or at least devote a whole episode around Sylveon, and they couldn't have done either one by that point. They also probably wanted to keep Sylveon's announcement a secret until they revealed its presence in the short for the next movie due to showcasing all of the Eevee evolutions and they'd probably want to keep its method of evolution, along with its type, a secret to create more hype. There wasn't much of a chance for Virgil to get a Sylveon right after X/Y were announced.Well, no, but I do remember that Virgil was due to appear in the episode after Sylveon was announced, and the animé has showcased Pokémon that haven't been announced yet. So, while Virgil could've had an Eevee in his first appearance, by the time of his appearance after X/Y's announcement they should've given him a Sylveon- since we all know that there's a certain level of communication between the animé teams and the game teams that at least allowed Newtwo to be showcased in the next movie.