So I think now that the cover story has ended it's safe to say that Vivi is going to be the last person to board the Thousand Sunny before the Straw Hats set sail for Laugh Tale.
The panel below is a pretty big indication that she is more than likely going to either reunite with the Straw Hats at the end of the Elbaf or at the beginning of the next arc.
I think if you go back and look at the ending of Wano you can kind of pin point the moment that Oda was starting to have second thoughts about adding Yamato to the crew.
I also don't think that this is the first time he's changed his mind about adding someone to the Straw Hats.
And I also don't think it'll be the last...
But don't fear Yamato fans! We'll definitely be seeing him and all the other Red Scabbards again when they join the Grand Fleet, Carrot and the Minks, Loki and the Giants, Gaimon, and many others allies to help the Straw Hats in the final battle against Imu.