You just lost all the benefits of Hayward's playmaking in the starting lineup
Vucevic is an excellent passer at the C position. There are only 2 Centers around the league averaging more assists than him: Jokic and Marc Gasol.
Hayward is averaging more assists than Vucevic, but he's committing more turnovers as well. If you combine assists + turnovers in one stat, then Vucevic is a (marginally) better passer than Hayward this season.
Vucevic's assist to t/o ratio is 2.62
Hayward's assist to t/o ratio is 2.56
gained the marginal benefits of Vucevic at C over Theis
(...) you maybe, possibly got a small upgrade at the 5
In what universe is Vucevic a ''maybe, possibly small upgrade'' over Theis?
Vuc was an All-Star last season. This season, he is averaging 19 points, 11 boards, 3.4 assists, 1 block and 1 steal per game.
Theis was our third choice Center last season. Only reason he's starting this season is that Danny used our cap space on Kemba (=Kyrie replacement) and we had no cap space left for a Center (=Horford + Baynes replacement). To put it another way, starting Theis at the 5 was never our plan A. We got stuck with him. Acting like Vucevic is a marginal upgrade over Theis, is homerism at its finest!
Vuc is a good shooter, passer, and rebounder, but I wouldn't say he is a great defender, which would be the main issue with your proposal.
Well, Hayward isn't exactly ''a great defender'' either (especially after his injury). We would have to pair Vucevic with a defensive-minded PF, which is fine cause we already got Theis, Semi and G-Will.