So here's an answer Mabula gave in the past and works for me. Though it's always preferred to use real flats to correct vignetting;

On all lights if you make an artificial master flat, select "create artificial flat". This will save the correction model as an artificial Master Flat. You can then use this Master Flat, just like an original master flat to correct the vignetting.

To get the most accurate result, first calibrate the lights with a master dark or master bias (create masters & assign to lights button, tab 2). Save these calibrated files (tab 2 all the way down). Then clear APP or reload APP and make the artificial flat using one of the calibrated frames (load in 1 of the calibrated lights). Then load in all the calibrated lights and add the artificial flat, calibrate again and see if that works.