Summary: When "Vertical Tabs" is on, the "List All Tabs" button should be in the sidebar with the vertical tabs, rather than in the main browser toolbar.
The "List All Tabs" button is located at the end of the Tabs row when Horizontal Tabs are used. This makes sense. (It appears as a downward chevron in this configuration.)
When Vertical Tabs are on, the "List All Tabs" button goes into the main browser toolbar. (It's the middle one here; different icon image for some reason.)
But that puts it fully on the opposite side of the window as the Vertical Tabs, which default to the left sidebar. I was looking for this button when using Vertical Tabs and couldn't find it. I expected it to be with the other icons at the bottom of the Vertical Tabs sidebar:
It seems more intuitive to me to put the List All Tabs button down here, or below the "Open a New Tab" button, rather than in the main toolbar at upper right.