Origins of The Great Black Mage. *SPOILERS*

[Kawaii Ryûkishi]

It's not at all insinuated - it's right there in a hidden cut-scene at Quan's home. Quan was out on his pier 'fishing' into the Mist one day, and his line snagged Vivi, presumably off a passing airship as Kishi says.

I remember that now. Man, I love ATEs.