The vignetting in my camera viewport and final renders are very different – see attached screen shot. It seems as if the dark vignetting in the corners are appearing at the bottom of the entire viewport rather than the corners of just the camera area. But when rendered, the vignetting does cover the actual camera area. This makes the final render much darker then when setting up the scene via camera viewport.
Now, my camera does have several elements animated, such as Focal length and Shift X. But even after deleting those animations the issue persists.