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.