He had a great season with Sevilla and he played quite a bit of RB for them during the 2014/15 season. But that was the first season he actually played RB for them and while he did play well when he was asked to that doesn't mean it will always translate to success.
What happened was we bought into the idea he could replace Alves as our full time RB. when we made the signing there was a lot of doubt over Alves future but he eventually signed a new deal only to leave a year later.
Lucho liked Hard workers and Vidal was one. but like only after like two months after he finally was able to play, Lucho stopped using him as there were rumors he got injured doing something stupid on the training ground.
He got another chance during the pre-season of 2016/17 but didn't do much to cement his place as Roberto ended up being the superior option. He had a dreadful game vs Alaves, well to be fair he wasn't the only one that had a poor performance, but he got sent to the dog house by Lucho.
He came back and several decent cameos only to get his ankle Butchered by Theo Hernandez. It's fair to say this signing failed as he never could make the transitions from Winger to RB, and all he is now is a Winger again and not exactly doing a great job there either.
We are fortunate however we finally have two good RB options now and a young Dembele coming back soon. Vidal is pretty much a fringe player now.