This portrait is framed slightly more tightly than I am comfortable with when using a 55mm lens on a full frame body. Because of the close perspective, the features closest to the camera begin to appear too large in relation to the not-so-close features. My suggestion is to move back and use a longer focal length for head shot portraits.
The vignetting present in this image is as out of the camera. With the subject's face in the center of the frame, the vignetting helps draw the viewer's eye to where you want it to go.
Camera and Lens Settings
55mm f/1.4 1/160s
5760 x 3840px