Amal

It's unlikely at this point that Vivi will become a Strawhat for 3 reasons, even though we love her:

1. She has no 'role' on the ship. During the Alabasta arc she served as an informant of sorts, providing the crew with information about the grand line and Baroque works, but that role is now filled by characters like Robin and Jimbe. Vivi's knowledge is too limited compared to these characters with her having no understanding of New World islands or politics.

2.Vivi doesn't have the necessary narrative qualifications to be a Strawhat. Although she's a great character, she's limited by both her lack of useful abilities and the fact that she doesn't have an overarching goal that would require her to travel with the crew. Even if the plot develops her a reason to reunite with the others, it's impossible for her to develop the necessary strength to be on an emperor's crew in such little time. Even if she gets a devil fruit, it's unlikely that Oda would power her up enough so quickly or spend too much narrative time showing her powering up in the crucial final saga. It took all the Strawhats arcs upon arcs and meeting new people and coming across new power systems and tools to slowly increase their strength so it wouldn't make sense for One piece to throw away it's power scaling just for her. And unlike the less powerful Strawhats like Nami or Chopper (who are still wayyy more powerful than most people) she doesn't have a skill that would justify her presence in the plot despite her low fighting skills i.e. Nami makes up for her lack of combat by her intelligence and navigating skills, Chopper his medical expertise, Robin her ability to read Poneglyphs, etc.

3. Vivi has her own journey and storyline. Her importance to the plot shouldn't be dependent on whether she joins the Strawhats as that would take away a crucial perspective into things like the void century, the D clan, etc. That the Strawhats may not come across.