I think she's more level-headed than Nami, and what with Nami's recent greed flanderization, having someone normal like Vivi on the crew again would be nice. Plus, her shortcomings could change if we see more of her. As for fighting skills, may have been pretty weak before the TS by Straw Hat standards, but she was still able to hold her own and become one of the top assassins in a vast league of bounty hunters.
If a hakiless bum like Croc can go from getting his ass handed to him by a hakiless east blue sea monkey with a little blood on his fists, to a sudden top tier facing Doffy and Whitebeard like nothing after LOda's asspull writing a few months in prison, I don't see why Vivi can't get something decent after two years. At the very least a DF should balance her out. Plus, her family line clearly is heavily tied to the story and Imu's origins now, and her country will not know peace from the WG until Imu is taken down.
Loki though seems like the better fit right now. The only problem is that he's so strong that all other Straw Hats beneath the M3 will seem irrelevant by comparison, especially Bumsopp and Product Placement Chopper who are just... there.