When I try to put an NSTabView inside a vibrant dark Popover (as an NSPopover contentViewController's view) I get a completely black background. When I put it in a regular vibrant dark window, it works. When I put it in a vibrant light popover or window it also works.
Here's a screenshot of the result:
As you can see, the controls inside "go through" the black background and show the vibrancy.