you could use a Babylon layer for this
https://www.babylonjs-playground.com/#08A2BS#1 the last parameter is changing from background to foreground.
If you only rely on a vignette effect you can also enable it on any standardMat or PBRMat with:
scene.imageProcessingConfiguration.vignetteEnabled = true;
and configure with all properties:
scene.imageProcessingConfiguration.vignetteXXX