Why dark vignette looks posterized? I've been trying to render a scene with a dark infinite background, but when I use a vignette effect it looks posterized. I thought that it might be something I did in my Blender file, but then I started a new file just to test it and it seems like that's the way it works...
Bright background (no posterize effect)

Dark background (posterized)
Open the image to visualize it better

I tried adding a blur filter but it makes it worst:

Blend file: https://drive.google.com/file/d/1pIytoPHVPMyP7YvvJW72glZ9j9wP4Kz5/view?usp=sharing
What you are experiencing is a quantization error called banding.
Most monitor can only display 8 bits per color or 256 tones from dark to bright. There are just not enough levels to do a subtle gradient between two dark tones or similar colors.
It is a limitation that will show in most monitors (unless you have one that can do 10 bit display).
As a workaround, you can play with the dither settings in the Post Processing settings for the scene.
(from the blender Manual)
Dithering

Dithering is a technique for blurring pixels to prevent banding that is seen in areas of gradients, where stair-stepping appears between colors. Banding artifacts are more noticeable when gradients are longer, or less steep. Dithering was developed for graphics with low bit depths, meaning they had a limited range of possible colors.