Who do you think is better?

waw are you 2 serious now or what?

^Not everyone understands sarcasm....

anyways, they are very different to compare. Vidal was waaay better than Modric last season. This season Modric has been doing great, better than Vidal who imo has been underrated.
Modric is a natural attacking midfielder who was forced to play a lot more defensive than he was used to and has recently performed great. In the other hand, Vidal is a natural defensive midfielder who has been given the opportunity of playing as offensive as any striker in Juventus, being among the top scorers of the club since he got there. Vidal even used to play as CB with Chile.
In the end there is no better player, just a player more suitable for a team than another.. Modric has been doing better this season but I would rather have Vidal at United.