Reviews are welcome here, and so are bug reports or feature requests (preferably by mail so I can answer/ask for more info); I usually respond quickly to those, and indeed a lot of the features were added because somebody asked. My e-mail is provided in the add-on info.
If you are an extension developer, you are also welcome to work on the code and send patches. There is a git repository of the add-on at http://repo.or.cz/w/verttabbar.git
Since version 2.4 the add-on has localization support (thanks to Goofy of BabelZilla), so translations are welcome.
You can send me patches or even send me your repo.or.cz user id if you want to have commit access.
Next major version (3.0) will introduce a way to toggle between vertical bar mode, and standard bar mode without a need for browser restart. I'm not really sure of the UI I will provide, but I'd like to have an "all windows toggle" and a "only this window toggle".
Don't expect 3.0 soon, I'm really busy currently.
On the other hand, there WILL be 2.x versions for bug fixes and not too big feature requests. I really want to give to you the add on you want, so keep the wishes and bug reports coming (by email please, even if you also put a review; I'm too busy to check often for new reviews).