VV can be a bit flakey on the best of systems.
One way to guarantee to get it working is as follows (a workaround at best!)
Export all of your backing tracks to a standard wav
Open up a brand new project and import the backing track
With both projects open, drag/copy the tracks you want to apply VV to into the new project on separate tracks
Do all of your VV work, bounce each one to clip, then drag back to the original project