Removing some reds from the shadows in my final Lightroom edit helps a little, but I would like to understand what is causing this vignetting and how to get rid of it earlier in the process.
NLP settings used for the sample shot:
LAB Soft
WB Auto-Mix
All other settings at default
Having a red, bright vignette after inversion means that there is regular (dark) vignetting in the digitization. When the negative is converted, the vignetting gets inverted and creates a tint (because of the negative mask).
A few things to try:
Make sure your aperture on your lens is high enough to avoid vignetting. This will vary a bit by lens, but I usually recommend f 8.0 or higher to avoid vignetting.
You can go into the “Lens Correction” section, go to “manual” and adjust the vignetting. Do this on the original RAW file (not on a copy) and when you get this right, it should correct not just the brightness, but also fix the redness. If you need more control over the shape of the vignetting, you can also play around with gradient masks, as shown here: Lightroom Tips & Tricks | Negative Lab Pro