V-Vocal problem: sound is phasey after bouncing tracks; Sonar Producer 8.3.1

Hi Guys,
Not sure what's happening here, but it seems that when I bounce a mix that has a lead vocal track and a harmony track (that have both been V-Vocal'd ), the result is not pleasant...   the resulting vocals sound very phasey and artificial.  However,.... When I play the whole set of individual tracks within Sonar, before bouncing, including the V-Vocal'd tracks, it all sounds fine.  But after bouncing, the vocals sound bad.  I select the 64 bit engine, and shut down all other options for the bounce.  Can anyone tell me what's happening here ?   Can you not bounce multiple v-vocal'd tracks and expect something good ?
Thanks much,
Chevy