So... how did Vivi get so many kids?

[Wolf Kanno]

And once again, no one is explaining how we just automatically "know" that Vivi is dead...

The final monologue at the game's ending is Vivi's farewell speech Zidane and the party. It's more obvious in the Japanese because the speaker uses a speech pattern exclusive to Vivi.

Not to mention he never shows up properly in the ending like the other characters do, only his kids.