A background vignette is a gradual darkening of the edges of an image, which can be used to draw attention to the central subject. It is a popular technique in portrait photography, as it can help to create a sense of depth and intimacy. In Photoshop, there are several ways to add a background vignette, but the most common method is to use the Lens Blur filter.
To add a background vignette using the Lens Blur filter, first open your image in Photoshop. Then, go to Filter > Blur > Lens Blur. In the Lens Blur dialog box, adjust the settings to your liking. The Amount slider controls the strength of the blur, while the Radius slider controls the size of the blurred area. The Center slider allows you to position the center of the blur, and the Edge slider controls the softness of the vignette.