For portraits IMHO the most important element is the model's attitude and pose comes next, lighting whether it is ambient light or the way you use the flash, all the rest are simple details.
Vignetting can be creative if is gradual, however black corners aren't something you'll enjoy seeing.
Not always I pud my subject at the center of the frame, so corners performance is important however you don't always need high sharpness except if are shooting for example elderly people and you want to show every wrinkle
Vignetting can be creative if is gradual, however black corners aren't something you'll enjoy seeing.
Not always I pud my subject at the center of the frame, so corners performance is important however you don't always need high sharpness except if are shooting for example elderly people and you want to show every wrinkle