Currently, Corona does not consider Optical Vignetting option from PhysicalCamera.
If you want to use vignetting, you can either use directly Corona vignetting (which can be found under tone mapping option, directly in Corona frame buffer) or
you can enable vignetting in PhysicalCamera (in rollout Exposure), changing the exposure vignetting in PhysicalCamera, will result in direct change of vignetting in Corona frame buffer.