Vignettes have the power to direct the eye more dramatically than any other method-
until they're noticeable.
Then they're shlock.
example of bad vignettes: Wherever You Go
I shot this through a canon AE1 camera body with a badly vignietted focusing screen.
But with the ability to track windows (attach them to elements within the shot) and animate opacity, a good colorist can make them invisible. This is one of my favorite features in DaVinci Resolve.
To get the look right in this spot, I needed some power windows to bring down the background and focus the shots. With a static camera on a tripod this would be really simple, but some of these are handheld. So as soon as the camera bumps around organically it looks like somebody's smudged ink on the lens.
To marry them to the shots I had to track the backgrounds, but the default tracker sets points inside the window you've created. If an arm moves across any point, it starts tracking their hand instead of the background and you've lost it. So I needed to set my own points on the background where no arms or heads would cross.
Once the windows were tracked to the background I feathered off the edges and opacity to hide them. Whenever you start creating artificial lighting, subtlety is key. I always end up lowering my key mixing gain(or opacity in english) once I've got an effect working the way I want through a shot. Its how I lower a node's impact without messing up other settings.
I've also got highlight punches pulled on most of the character's faces to draw attention to their reactions in the wider shots and to make the overall blue gray look match without effecting skin tone and hair color.
Here's one of the referance shots to compare