The Fuji X100S camera already had a built-in, always-on lens profile that corrected vignetting and chromatic aberration but not geometric distortion.
Now, with ACR 8.4 RC, there is an external lens profile that corrects vignetting (again) and geometric distortion. This results in double-vignetting correction.
Here is a side-by-side that shows the vignetting correction being applied back in LR 5.2 with the built-in profile:
Here is a side-by-side in the new ACR 5.4 RC that show additional vignetting correction occurring along with the minimal geometric distortion correction as performed by the new, external lens profile:
This extra vignetting correction can be removed by setting the Vignetting slider amount to 0, but the user should not have to do that for each image, or save the corrections as new Adobe defaults just to make things work correctly.
One would hope that Adobe could add an internal Vignetting slider bias of -100 so having the Vignetting slider set at 100 results in the correct operation, but the user can still select 0 to undercorrect or +200 to overcorrect the vignetting. As it is, now, setting the Vignetting slider to 0, this only removes the overcorrection, rather than removing the correction entirely.
Another solution would be to strip the vignetting correction from the new X100S lens profile, but one reason not to do this would be that some third-party software also uses Adobe's lens profiles, and if the vignetting was removed, then those third-party software wouldn't have complete information for doing their lens corrections.