It's fireboard, renamed. There may be some other differences as well, but not huge ones. Possible the fireboard import tool in Impex might work.
Read something similar at https://www.vbulletin.com/forum/show...e-to-vBulletin --- It may not be a bad idea to try as a test shot in the dark perhaps LOL - but definitely not something that's guarenteed to work, as fireboard and kunena are not the same applications. And how do you know the differences aren't huge?!? That's a hypothesis right?...
Personally - I'd still go for the kunena ==> phpBB ==> vBulletin route, because such a method is proven to import data without issue.