I must admit that I don't have a lot of experience with this as Its very rare for me to ever need to display text vertically. Looking at the requirements of displaying text vertically i'd say that unless you have some specific niche reason for maintaining compatibility with iOS 3 I'd honestly shoot for being an iOS 5+ app, assuming that this is going to be a major part of the app because I don't think it has been till just recently in OS X 10.7 and iOS 5 that doing vertical text was made easy.

The further back you go in iOS the harder it is to do things like this and there a lot fewer users running older versions of iOS. And when you eventually drop support for older os's its just going to take a lot of work to change from 3rd party libraries (assuming there are any) to better things like Core Text. In the long run its just going to be more worth it to just use iOS 5+ api's to do this. If you really need to maintain compatibility with iOS 3 then I might ask StackOverflow this question because I am really unsure if there is any means to do this well while maintaining api compatibility.

All I can say is that personally if I was handed this requirement of displaying lots of vertical japanese text I wouldn't hesitate to make it iOS 5+ only and probably go the Core Text route first. There are plenty of users on iOS 5 and writing & maintaining the app will be a lot easier.

I hope this has been of some help.