Audio anomalies using VocalSync - and the cause

For the first time, I tried VocalSync yesterday. Twin guitar licks were the target (I do mostly instrumental work), which is why it has taken until now. As I let Platinum start playing the track so I could run the control knob, a loud and rhythmic popping started right when play arrived at the matched audio. After stopping the transport, the sound continued and eventually faded to nothing, but it would happen each time I tried to play it.
 
Then I decided to turn off the VST's in each track. That was the problem. Both tracks had the same GTR preset in place, and without them the track played normally. If I could hear the guitars as clearly without the VST, this would be fine.
 
Has anyone experienced this with VocalSync?