Vocalsync Keeps Failing

Are you talking about the message that pops saying that the guide track is empty?
 
Other than that, Vocalsync is just a nightmare in general.  Very problematic.  I was really excited about this feature but it has been almost unusable due to the HUGE issue that I and many others have run into that causes deafening white noise when you play a clip with Vocalsync enabled.  I've almost went deaf and almost blown speakers a couple of times b/c of this. Smh. 

A good alternative to VocalSync is to use the "multi-track viewing" in Melodyne

I have recently done this by adding a Melodyne region fx to all of the vocals

You can currently still only work (edit) one track at a time but you can see the other tracks, which makes it very useful for lining up timing of words (and key syllables) as well as checking on the tuning of harmonies

It is obviously best to do things manually but there are auto-correct options in Melodyne with the a selection of the % correction so you may find a good balance for an auto option

I now much prefer tightening timing issues with Melodyne than audio snap for vocals - but tend to still use audio snap for drums to keep the workflow going

In situations like this, it's always worth your while doing a bounce to clip before invoking any of the more exotic processes we have available.