It is an old bug in Sony Vegas. One of those, which are very difficult to reproduce.
And I think that this happens from time to time with anyone who has the toolbar icons from different extensions.
Vegasaur Voice Over tool can lower the volume of multiple tracks with a few clicks (just select the desired Target Track and click Apply, repeat if needed)
For the best results you need a motion tracking plugin (this is not the same as Track Motion in Vegas Pro). Something like Corner Pins in Boris Continuum Complete.
In the past, DualEyes could do what you want, now you need PluralEyes 3, which has integrated DualEyes features. Free trial is available, so you can try it