There are 2 different ways to pan-scan in Vegas. You can do it with the Track Motion that affects the whole track, and you can do it with the Crop tool on each event. I’m not sure how you are doing it, but if you first do your multicam edit, then expand out to multiple tracks, you should be able to use the track motion feature on that camera’s track.
If that doesn’t work, I’d do your pan and scanning in a separate project, render it out and bring it back in.
btw, if the widescreen footage was shot in HD, I’d crop the HD to 4:3 to match the other 4:3 rather than doing it vice versa. Cropping 4:3 SD to widescreen in post is pretty ugly..
hope that helps. good luck.