I have been RawTherapee 5.9 Dev for a while, and one of the biggest additions to it is Local Adjustments. It recently occurred to me that they can be used to create a vignette centered anywhere on an image.
DSCF5819.RAF (21.3 MB)
DSCF5819-vignette.jpg.out.pp3 (16.7 KB)
CC BY-NC-SA 4.0
My starting point was neutral + auto-matched tone curve.
Step 1: Lower the exposure compensation.
Step 2: Add a local spot centered where you want the vignette centered. Add the Dynamic Range & Exposure tool and set the scope to 100 (this will allow the spot to affect all the image regardless of color or tone). Raise the Exposure Compensation to the to the same value that you lowered the whole image.
Step 3: Make sure the Show Additional Settings  box is checked. The Transition Value slider will function akin to a feather or radius control. The Transition Decay slider will function akin to a strength control.
Step 4: Play with the shape and size of the spot to your liking.