I'm posting this here for visibility rather than in Town Hall.
vBulletin as a company is in a tailspin. If I upgrade to the vB5, I think most users will hate it (that's what's happened on most sites that have converted) and things like the high five and MHS will go away. At the same time, It's getting harder and harder to stay on the old version. Their mobile apps aren't very good and hard to work with. Forum Runner will not get any future updates (it was bought by vBulletin's parent company, and then development was killed on it).
One option is Xenforo, which was created by some of the original vBulletin designers. They don't have mobile apps, but they designed it to be responsive, so it works very well on mobile phones. Check out their support forum here, and you can even easily login to it using Facebook, Twitter or Google (or register an account) to see how posting, thanks (what would be come a high five) and the like will work.
Please visit
https://xenforo.com/community/ from your phone/tablet and see what you think, and if this would be a reasonable alternative to a mobile app.
Based on playing around, I think Xenforo has done such a good job with their reponsive design (making it work equally well on PC, tablet or phone) that I don't think a mobile app is necessary.
Please take a look and give me your feedback or any other thoughts on the subject of vBulletin, moving and the like.
Thanks
T