Good day. Disclaimer: I am no pro with cameras.
The vignette is caused by the barrel of the lens.
I would normally fix my aperture at F8.
I learnt this from some one in this forum many years ago.

If I zoom out even further say 300mm to 400mm I would select F9/F10.
To add, vignette can get worse on low light days....increase to F9/F10 & so on, on these days.

You can google cause of vignette to get a better explanation....