In regards to Vivi, I've heard people voice that she wouldn't put her desire to rejoin the Straw Hats over the people of Alabasta. And yeah, I can totally get that.
Here's the thing though, in light of the recent events, she might not even have the choice to be a proper leader for the people.
Recall when the Five Elders met with King Im (Ch 908). He was holding a picture of Vivi as they asked Im to reveal the name of the "light" that needed to be extinguished from history.
One possible interpretation I have is that Im wants Vivi for some reason, but she's untouchable due to her status as royalty. So in order to have her captured, he needed to 'extinguish' the Nefertari's royal status, essentially making her no longer a princess (perhaps having her go AWOL in the wake of Cobra's death would cause this) .
She'd realize she couldn't fix things for the world in her old position, so she rejoins the Straw Hats in order to bring the world to the vision she wants.