A little late to this discussion as I'm a new user of Orion, but I also do find it very confusing that there is no indication of where you are in your list of (vertical) tabs.
I'm a designer, so I can't help myself, so my thought was: when you have vertical tabs and sidebar closed, the tab list could be in a sort of tooltip which shows when you are switching tabs via keyboard for example. (think like the command-tab app switching view)
Additionally, perhaps in the case where you have named the window, it could show [window name] | [# tabs] so you have some indication that there are more tabs, with a way to hover over to pop open the tooltip and switch tabs quickly, without opening the sidebar. (I'll leave it to you to figure out whether to have this be part of the existing Window tooltip or a second tooltip, or what.)