The first thing you need to do in the Lens Correction dialog window is switch from the ‘Auto Correction‘ tab to the ‘Custom‘ tab – right below the Cancel button at the top.
Under the Custom tab, you’ll find the Vignette settings.
Here you can adjust the Amount of the vignette. Moving the slider down to the left will create a darker vignette. Moving it up to the right will create a lighter vignette.
Right below you can tweak the Midpoint. The Midpoint settings determine the size of your vignette.
Moving the slider down to the left will narrow down the focus point and increase the size of the vignette. Conversely, moving the slider up to the right will decrease the size of the vignette and enlarge the midpoint.
Play around with the settings to see what you like best. Once you’re happy with the result, click the OK button.