Can't figure this one out.  Prior to Vocalsync region creation, the clip in question plays normally - it is an doubling-track to the lead vocal.  When I initiate the regionFX the clip then produces no sound, and if I try to render the clip, I get a fatal crash.  The clips reference is set to the lead vocal.  Any ideas?

Apparently if I export the clip to a wave editor, make some minor change to it (silence a bit of breath) save it, and then re-import it back into the project, vocalsync works OK.  Huh?  Anywho, I have a workaround.

Is the clip in a Take Lane?

Yeah, take lane.  That is apparently the problem.
 
The solution (I think) is to stick with what I recommended in another post, which is to use a temporary track to comp or autopunch-takes, then copy the result of that into my "keeper" track.  I guess I got carried away with the fleeting moment of finspiration, and set up a loop with an autopunch inside on my "keeper" track instead of on a dummy track ... and then I couldn't figure out why Vocalsync was being evil.