For portrait use, vignetting is usually a minor concern rather than a deal-breaker. Many photographers actually like a little corner darkening because it can help draw attention to the subject.

The main downsides are consistency and flexibility. If the vignetting is built into every shot, you can’t remove it later as cleanly as adding it in post when you want it. It can also become awkward if you crop off-center or reframe heavily, because the darkening may no longer sit in the “right” place.

If you shoot RAW, vignetting is often easy to correct in software, and for many lenses the correction is straightforward. Mild vignetting is often barely noticeable; stronger vignetting can be more of a concern because correcting it may affect image quality more.

So for fast portrait primes, vignetting usually shouldn’t be the deciding factor. It’s common with wide-aperture lenses, and you’ll often be choosing between different amounts of it rather than avoiding it entirely. In practice, factors like aperture, sharpness, and overall rendering are usually more important.