I don't understand the people who refer to Vucevic as "mediocre." He's a center who averaged 21/12 with 3.8 ast last year, and even began to make threes at an acceptable (36%) rate. Not great defensively, but does not seem to be a liabilty any more.
He would be an improvement on Al in several ways (not defensively).
You can be a great offensive player but still just a mediocre overall player if your defense gives back most of what you contributed to on the offensive end.
I am not say Vucevic is mediocre, I think he is a very good center, overall, but I am not someone who loves bigs who are bad defenders, so I wouldn't be committing huge money for him on a 4 year deal.
I would much rather use the cap space to absorb a Steven Adams for two years. Great defender and rebounder that won't demand shots that should be taken by Hayward, Brown and Tatum so they can develop their games. And, the contract ends with Hayward's contract, possibly clearing space for a free agent.