Di Natale moves in behind the opposition defence very early and looks to stretch the game when there is space to do so.
Neither Vucinic nor Jovetic are naturally going to run in behind a defence early. Vucinic is more inclined to drift wide and
Jovetic will offer himself short and look to get involved with linking the play.
When that space is not there, Udinese tend to rely on penetrating runs from deeper areas from midfielders or wing-backs in order to break a deep, compact defence. Di Natale is then focused primarily on being in goal-scoring positions so that he can finish off the moves that got those runners from deep in behind the defence.
Neither Vucinic nor Jovetic are naturally going to get into goal-scoring positions on a consistent basis and neither are going to finish as consistently well as Di Natale does.
In short, Juve really ought to be focused on getting a proper goal-scoring CF.