Really I'm expecting Virgil's Eevee to be something of a mimic of Ash's Pikachu in the respect that it probably won't evolve, it's just an unevolved partner that doesn't want to evolve and is just as strong without needing to evolve. I wouldn't even be surprise if the final match came down to Eevee VS Pikachu for that somewhat epic feel of the two weaker Pokémon battling for such an important match, but I think that if it was going to evolve into another eeveelution then there would've been some kind of feature about it like a silhouette or something since they would want as much hype as possible surrounding it.
As for who'll win and how far Ash will get, that's actually a really hard call to make with Unova. Since they wanted to make it so different to the previous sagas, I honestly wouldn't be surprised if he actually went on to win this one. But I'm betting on either Ash or Stephan winning; Bianca would be my third choice but I really don't think she's mature enough and fully developed enough as a character to win a league yet. I don't think it would be a problem if Ash was to win though since they can easily still have him go to another region to compete in their league too; if anything it'll just be a joke for them to play on when he's acting like a noob in the first few episodes like he always does; it'd just be refreshing to see him actually win something important in the Unova saga haha. I suppose it's also possible that someone else will win and they'll just stay in Unova till the next Pokémon league takes place to fill the time until a new region comes out, but I really doubt that. Maybe they'll also go for an Orange Islands type of adventure, who knows :o Or if he does win then there could be a conference/tournament for all recent league winners to take part in to find the best of the best and he could enter that, or maybe he'll even face Unova's E4? So many possibilities haha.
To be honest though, I've been expecting the "Team Plasma is going to step in and cancel the league, creating a new arc to keep Ash in Unova a little while longer" to happen from the start based on what happened in-game and Team Plasma's lack of appearance so far in the anime, but who knows, maybe they'll just wait till after the league to introduce them. It just seems really early for the Unova league to be happening already so I don't think it'll really go ahead straight through till the end. (I haven't read many spoilers or stuff yet so if this has already been disproved, sorry!)