ah..well my 2nd point stands. "why"
Well, I can't speak in Fergie's name and tell you why
Personally I think that Vucinic impressed Manchester back in the days when they played Roma. That's why they are linked to him every single summer since 2008. Then he played an absolutely brilliant match against Chelsea in CL and, as you know, CL is one big advertising for players. England are in the same group with Montenegro in the EC qualifiers and surprisingly the Montenegrins, led by Vucinic who in Jovetic's absence is a one man team, are doing great and they are sharing the 1st place with England (they play England in Montenegro after getting a point in England).
So, more or less, whenever United and English fans had the chance to watch him, Vucinic shined. He's highly rated there (Opposite to Zlatan who choked in most matches against English teams).
Now, as for where would he play, I believe that they plan him to be Berba's replacement. There were loud voices that Berba will leave United, and I wouldn't be surprised if he does. He is 30, almost three years older than Vucinic, he's high earner and he lost his place to Hernandez.
Of course, all the above is nothing more but loud thinking from my side.