That's because Osvaldo-Llorente doesn't work as a partnership, they both need a second striker with them.
Vucinic should be useful in the run in, provided he stays fit.
With Vucinic, the gap between the midfield and the attack disappears. On the other hand, he isn't a finisher, so with him we create much more and score less, while without him we create less and score more. Perhaps the numbers won't back me up on this, but I have the impression that last year we created much more chances. Almost every second match felt like this year's match in Kobenhavn, where we created 5-6 good chances for 1 goal. Now we're more lethal, but we struggle to create good chances. The goals we score are mostly about power, deflected ball etc.
Of course, the goals are what counts, but the thing Mirko offers is something we badly lack, especially against the good teams and in Europe. It even makes one wonder how far can this team get with an AMC. It's something we won't try, especially after the failed Diego experiment, but we can't deny that a creative AMC would solve lots of problems in our play.
Unfortunately, our strikers are far from complete. What Llorente gives in the box with his physical power, is what we need, but noone else gives. What Mirko has just outside the box is what we badly need, but only he has it. Tevez's goals and work rate are something we can't do without. But we can't play the three of them without ruining our midfield, and only two of them don't cover everything in our attack.
Anyway, if it's between Mirko and Llorente, I have to point out that Llorente scored 13 goals in 2500 minutes this season, while Mirko scored 14 in 2800 minutes last year. Perhaps what's supposed to be Llorente's greatest advantage over Vucinic, isn't such a big advantage, while what Mirko gives in the link-up (the fullbacks and the midfielders are playing better and they become more involved when Vucinic plays) is something that is by far more important than those 1-2 goals Llorente scores more than Mirko, and his fights in the box when he tires the defenders.