In terms of Vivi joining, she has always been an option, even if she is far behind strength wise and such or lacks a role.
Vivi has always been different then every tag along and treated different then everyone, she has stayed popular all throughout the series eversince Arabasta and is popular in both hemispheres.
Vivi has always been the closet to the Strawhats that no matter the distance or the passage of time she still remains imo the closest to the Strawhats above everyone they meet, when ever something happens to the Strawhats its Vivi thats close by, wasn't surprised at all by Robin joining, was deciphering Luffys message when he went to marineford again (considering she knew Robin join before it was really reported don't underestimate her), clearly still has the same spirit from Sailing and that no-nonsense attitude.
She is even developed different compared to the WG princesses just by rereading Arabasta not to long ago, active to take roles in participating in the Government from a young age, knowing the meaning of her actions with Wapol, taking charge to chase and infiltrate BW.
Now with Revivre, we learn/know Arabasta is treated differently then any of the other 20 Royal Families that formed the WG, confronted CP-0 not giving a rats ass, Im has plans for her as unlike dangerous people/targets like Luffy, Blackbeard and Shirahoshi that were torn up Vivi has left unscathed and just the news of Arabasta, hell Greg on twitter brought this up I think is worth mentioning:
I've never been concerned with Power as @robby says Oda can come up with whatever he wants to bridge the ower gap if he wants:
- Training with pell and co since the Timeskip
- Inheriting Kumas Fruit or some other fruit
- Training with the Revs if/when we learn there stories are linked together with the Revivre
Oda can come up with anything if he really wants to, I don't think anyone should dismiss Vivi as she is clearly treated differently then every other Princess or Strawhat Ally
Discount Vivi at your own expense.
Vivi also has a god tier unit in Treasure Cruise that atm will never be power crept out.