Yes. In Lightroom 4, presets aren’t edited directly in a separate editor—you usually apply the preset to a photo, change the settings you don’t want, then save that as a new preset or update the existing one.

For unwanted vignetting, check both of these panels in Develop:

• Effects
• Lens Corrections

Some presets apply darkening through Post-Crop Vignetting in Effects, while others may use Lens Corrections settings.

Workflow:

1. Apply the preset to an image.
2. In Develop, find and remove or reduce the vignette-related setting.
3. Save the result:
   • Click the "+" in the Presets panel to create a new preset, or
   • Right-click the preset and choose Update with Current Settings.

That will give you a version of the preset without the vignette.