To be clear, the "add" button at the bottom of the tabs that was hidden with overflow was never a feature. It was always a bug - one that received several bug reports. So moving the button to the immovable space at the bottom of a vertical column where it would always be visible (specifically requested in more than one report) was an effort to deal with that bug. I feel that was a non-optimum effort. While it did eliminate scrolling and searching for the button, it replaced it, in the case of low numbers of vertical tabs, with mouse movement. There's a better way to do it.
But in the meantime, as I say, I just use the double-click. This adds less than a tenth of a second (may even save time by not having to locate and precisely hit the button) to the new tab generation when there's empty space. The location of the new tab button saves time in all cases when there is no empty space.
I suspect an analog to the horizontal tab behavior will be developed.