"Goodnight."
"Tomorrow is an event."
"What's an event? Vivio"

I'm not entirely clear on what they're talking about.

not "What's an event?", but "What event?" In some sense, 「～ってなに？」 can be interpreted as "What is meant by '~'?"

0xCCBA696 said:
not "What's an event?", but "What event?" In some sense, 「～ってなに？」 can be interpreted as "What is meant by '~'?"

It goes both ways, really. Depends on context. Though I'm clueless on the meaning in this picture as well.

"event" typically means a doujin event.

Yep, and Roffle's interpretation is right.

Wasn't aware of that connotation. :) Akari, yeah I meant in this situation, not in general. The more common meaning would of course be "What is ~".