Mixed feelings about this one.
I like Vucinic and think he's a very useful player.
However, I doubt I'd be starting him ahead of Llorente or Tevez very often.
As for for Guarin, I'm somewhat undecided.
I liked the look of him at Porto and he did well when he first went to Inter, but I feel he's been pretty poor for them for quite a while now. Obviously you can put some of that down to Inter's poor collective performances, but Guarin's decision making really has been woeful at times. Mazzarri playing him higher up the field also hasn't helped. I also consider him a relatively lazy player, but that's no great concern because we all saw how Conte knocked Vucinic into shape after Vucinic had arrive with the 'lazy' tag.
This signing does suggest to me that Conte has decided he likes the power of Pogba and Vidal in the more advanced midfield roles, so wanted another player along those lines to be backup to them, so I'd imagine this will be likely to confirm Marchisio is headed for a deeper role most of the time.
Another midfielder was certainly needed one way or another.
I'd expect Juve to have to give Inter a little cash. I'd value Guarin at about 15m, while I think Vucinic is probably in the 10-12m range.
If Guarin shows up with the right attitude and he is coached well, he could prove to be a very useful player. Juve's very set system and tactics and movement could really suit him well because it means that the player jut has to follow instructions a lot of the time rather than having to think for himself too much. That could mask Guarin's lack of intelligence.