The link in their announcement thread goes to the vBulletin site. It makes sense - easy(ish) upgrade path and they've probably got a support agreement with them.
What link? I don't see any link to the vBulletin site on their announcement thread.
The mentioned Reactions feature (e.g. more than like/dislike but "Haha" etc as well) is a Vanilla Forums feature as is Jail, moreover http://forums.digitalspy.com
is showing a Vanilla Forums error page, and it previously 404'd. I also don't think @mentions are a part of VB5 whereas they are, again, a VF feature.
Furthermore the upgrade would likely be simpler than taking 72 hours if going from VB3-5, I believe they are doing a major migration to different software entirely, and ditching vBulletin completely. And rightly so IMO, v5 isn't that good from what I gather.
I still think Vanilla is what they're going with, based on the above. We will soon find out.