1. The whole point of vertical tabs is that horizontal browser space is "cheap." Why not keep the tab list visible? Having it be a dropdown adds an unnecessary step.
2. Horizontal (Sidebery style) pinned tabs that just show the site icon are much cleaner. I have a row (8-12 tabs depending on how wide I drag the tab bar)of frequently used sites pinned at the top of my tab list.
3. It doesn't look like you have right-click move to window functionality, which is critical for my workflow. If you add it, be sure to make it work with the Titler extension so you can work with window names.
I'm glad you didn't put energy into tab trees - I think they are a waste of space. I have a window for each general category of tabs.