I’ll give my thoughts on Vivi. I think for being a spoiled rich girl, she still has a real nice story & even when she was a child, she showed determination to take care of Alabasta. I understand why you think she isn’t part of the crew, but I thought this part was real cool & emotional:
I consider her a Straw Hat only because of the way Luffy is & because of this scene. Also, if she ever did join the group, she would be on the same class as Nami & Usopp & eventually would of gotten stronger.