Someone who pledges to sail under Luffy indefinitely as his subordinate and make getting him to laughtale their primary goal.
Vivi will likely join, but will be treated as a guest and not a subordinate like before, her primary goal will be to take down the WG not to help Luffy, and she will have to preface it with "just until it's safe to return to Alabasta.
What do you see as the difference between Vivi and Kinemon, Momo, or Law if she only sails with them for one arc and then leaves? Surely you can't be saying just having a stronger connection with them makes her a Strawhat, because then Sabo is at least 90% probably joining too, likely Garp as well.
1.) kinemon motivation is momo, like he said he attack luffy if he hurt momo (no nakama material)
2.) Momo want to be the next leader and want luffy to stay with him in wano instead (no nakama material)
3.) Law is also want to find one piece, have friendly rivalry with luffy ( no nakama material)
4.) Despite wanna save her country,Vivi also want to see the world but she choose not too at that moment (end alabasta).her story could ended up there, but oda choose to make the promise scene and show her heart still longing for the sea (begining of reverie).
Remember, she is the base template for every female candidate we debate here (carrot/wanna see the world, yamato/stay behind for country, stussy/spy that work against evil org and bonney/want to save or revenge to goverment that killed her father)
and for sabo and garp, we both know that stupid and you just randomly throw that name for arguing sake. When garp say he want to do any pirate related stuff? And sabo? What make you say he has stronger relation ship with sh? Just cause he luffy brother? Just from percentage base, 7/10 current sh already met and talk with vivi but only 4/10 of sh met and talk with sabo. So yeah thats my answer.