Searching from tapatalk won't work, so it's been disabled.
Probably the simplest solution, though it would behave differently from the default vbulletin search, would be to change searches through tapatalk to use google.
A more complicated solution would be to get tapatalk's search to use the same sphinx search backend that the rest of vbulletin uses.
I'm thinking about buying tapatalk so I can work on one of those options. However, tapatalk needs to die, and vbulletin needs a nice mobile css skin. Unfortunately, unless there's a free one on vbulletin.org or something, that's probably more work than the alternatives of just fixing tapatalk search.
Imagine a pathologically idealistic, anti-religious, culturally-relativistic, iconoclastic, socially-irreverent, sociologically-progressive signature line here. Or don't.