I can't speak for Vault Wiki but I know a lot of coders interest in xF comes from their respect for Kier and Mike's work, not to mention how likable they are on a personal level. On top of that even though (for us at least) the number of new people buying vBulletin and looking for mods is still higher than the number of customers switching to xenForo, seeing loyal customers switch and ask you to please consider porting their favourite mods over makes you obliged to come and check things out. I'd imagine the Vault Wiki guys have also had a few loyal customers ask them to make their stuff for xF so they've come to take a look see.
A lot of our customers become more than just customers - they become friends and we like those guys and, you know, part of you wants to see if you can't make stuff they can keep using just because of that. Things are fine in the vBulletin market. DBTech had its best week ever exactly 4 weeks ago. Sorry to disappoint.