I've seen several comments online that the Vucevic trade was primarily about saving money, rather than winning. And, there's no denying that making the trade allowed the Celts to make further moves to get under the tax.
I think that the tax savings were an important, but secondary, consideration. I think Vuc is a very good fit for Joe's system, and a big part of this trade is hoping to resign him in the off-season at a reduced cost. We needed another center, and trading Simons now means we at least have a chance of retaining an asset going forward.
Yes, Mike Felger on Felger and Mazz has been beating this drum the past week, saying it was purely for money, the owner is cheap, etc. Decisions are rarely made for a singular reason as you alluded to, but I don't think they would have done it if they didn't think Vuc would add value. They had the KP TPE that Vuc fit into perfectly and his skill set also fit into the Cs offensive scheme pretty seamlessly. He's a stretch big that rebounds and spaces the floor, a KP lite from an offensive standpoint, that gives them a different look to the players they already have and Joe is huge on different lineups that create different challenges for the opposition to guard. I'm sure they knew of his defensive issues and already know how to incorporate the appropriate defensive schemes to try and mitigate as much of it as they can. And the money worked out, and he is an expiring where they have the chance to extend him for less money and play the Horford role, so all in all a win.