I heavily dislike the idea of Vivi joining the crew. Whether it was right after Alabasta or here near the end. I like Vivi, I really do, but she'd just be a burden tagalong that can't do anything to really help. I know neither Oda himself or Luffy cares about that sort of thing, but imagine a Straw Hat that can't fight anyone but the generic footsoldiers of the arc villain palling around with them. Like imagine the whole Water 7 Saga, Vivi would get one-shotted by Nero of all people. I've seen some people say she'd be the diplomat, the one able to get to a peaceful resolution, but that's never been a strong point of hers, and there's no major villain that'd listen to a word she says. I wouldn't be willing to believe the timeskip would help much with that either. Remember, she found Mr. 5 and Miss Valentine to be terrifying threats.
I don't even want any more Straw Hats, no matter who, to join at this point. It's too close to the end. I know that, again, Oda nor Luffy would care about this, but it'd feel almost like they cheated their way onto being a part of the King of the Pirate's crew. There can't be more than one, maybe two arcs between Elbaf and Laugh Tale, so it'd just be someone joining the crew and then two weeks later they're part of the Pirate King crew. Just feels...unearned. Especially if it'd be Vivi, who got to sit out 90% of the story.