Although I have seen this addressed in the Melodyne manual, I am not 100% clear what I need to do.  Here is the situation.
• I have four vocal tracks that have been extensively edited with Melodyne.
• I now want to change one bar of the music from 115 bpm to 98 and then back to 115.  In other words, one bar will be slower than the rest of the song.
• I did not know this at the time of the vocal editing.
• The voices are not singing during this bar although an end of a note carries over a bit into it.
 
My conern is that the vocals will be out-of-sync after the tempo change.
 
What do I need to do to put this one-bar tempo change into the project (song) without making the vocals slightly out-of-sync afterwards?
 
I know one option is to bounce the vocals, but I may do some more edits in Melodyne to tighten up the timing.

Insert Melodyne  then start the song at the very beginning let it play through the whole song ,, It will learn the tempo changes then you can cue up at just the beginning of your vocal track There is a little tab button next to the tempo window on Melodyne open it up and make sure it is indicating there is tempo changes ( the tab will slowley flash to indicate it saw tempo changes ,,,,  click to open) . then you can make Melodyne do its thing so you can start editing ..... I was having problems with tempo changes until I watched one of the tutorials at Celemony site.... Hope this helps.. It worked for me.

wouldn't rewiring Melodyne to Sonar (instead of VST plugin) be the better choice, due to tempo sync and cycle sync and playback?

There'd be difference in the way the vocals get to melodyne though: in VST mode, you just "record" (transfer) the vocals, with rewire, you'd have to export (from daw) and import (into melodyne) (or drag'n drop - and align)...