I've tried it on 2 different projects, on multiple tracks. Stereo BGVs.I select a clip (not frozen), right click, region fx/Vocal Sync/ create region fx. It un-highlights the clip only. There are not multiple takes. Same for Melodyne. This has always worked very well for me on mono clips before today (never used on stereo track). It does work on my mono lead vox track. Has any one made this work with  a stereo track? Thanks

Hi. I am able to use both Vocal Sync, and Melodyne on stereo tracks. Have you tried to bounce the clip to itself before applying Region FX?
All the best.