vBulletin is cumbersome, outdated, hard to administer, and buggy. Discourse is open source, modern, and offers a much better UX.
vB provide data dumps, which we run through a customised importer script. You essentially need .json files and you're good to go.
Potentially a bit of both. This will be the third such migration that I've been a part of, and in both other cases we've seen an uptake in activity. It is naive to think that a platform change will cure all ills, but one of the biggest benefits of Discourse is timely desktop notifications, and flexible user preferences – all missing from vB.
To be fair, engagement here is healthy. It's retention that I'd like to work on. Our content (and support structure) has tended to aim towards people that are new to UX, but our early audience has moved past that stage and we don't do a lot to keep them around.