I reckon that you could boost his Balance further. He relies a lot on his strength rather than skill to get past players, although he can be lacking in one on one battles - hence why the comparisons to Vieira often fall flat. Perhaps an 85/86 value?
On the other hand, there can be a definite comparison to Vieira when it comes to Fer's positional sense. He's positionally sound when it comes to defending and his Defence value should definitely be raised from a 60. On the other hand, his versatlity has prevented him from being in one set position so an increase in his Defence could prove awkward. So I'm unsure on a specific value just yet.
He does cover massive chunks of the pitch and does an awful lot of running, quite typical of a youngster. But unlike most young players, he doesn't tire out quickly and doesn't show to much fatigue when he plays. Again, I'm thinking that this could - potentially - go to a similar value as his Balance.
I'm also unsure of his current TS value. Over the course of a long run he gets outpaced quite easily, proving not to be overly fast. Does the current 81 reflect that well?
His Response definitely needs an increase, though. He responds very quickly to sudden changes of tempo and reacts quickly to a nearby loose ball, although sometimes his lack of pace prevents him from getting there as fast as he would like.