Actually, i think youre underrating Quag here. I don't know if you were able to see the last few games that Quag played for us. You probably missed how dynamic he made the entire team going forward, playing from a much deeper position, mind you.
There is no doubt that Vucinic prefers playing the wider forward role, but anything in and around the box is Quag territory. The advantage he has over Vucinic is that he can do most things Vucinic can in build up play, and go a step further with his finishing. He is by far the better goalscorer of the two.
Quag doesn't have the skills to play the SS role?! Surprising you say that, because that's all he's been doing every time Conte puts him on the pitch, and he's just as devastating there. His passing, movement, intelligence off the ball, makes him perhaps the most complete striker we have on the team. Technically, it may be apples and oranges for some, but Quag in my book is every bit as skilful as Vucinic. In fact I'll go as far as saying that last year's Quag, is well ahead of this year's Vucinic all things considered.
Yes, his shots are not what we are used to seeing, but every time he has been on the pitch he has been a threat for the opponent and always looked dangerous moving forward. I completely disagree with the notion that he is nothing more than super sub material. Its really a matter of the system being deployed that may or may not suit a player. Vucinic could easily be benched by the likes of JoJo and even Rossi in the 433. Current form is not a yardstick to judge the overall capability of a player when one has seen him perform rungs above that. I don't know how much you've seen off him or Juventus this season, but he has tracked back and helped out in d a lot more than Vucinic ever does. The only thing Quag is missing right now is goalscoring confidence, but thankfully there are more attributes to his game that Conte believes he can use and it's there for all to see. Lacking 90 minutes is a matter of returning from injury, and that's not up for debate for any player, let alone Quag.