Alright, I finally went and installed WA5, and I see the problem with the "Modern" skin. Winamp is being a little rude here, it's aggressively trying to keep you from moving its windows offscreen, which is exactly what the VWM wants to do.

The Classic skin is a different matter... Their LiteStep preferences don't seem to have any effect at all (at least with RabidVWM), and the playlist seems to be refusing to move for some reason. I suspect it has something to do with the fact that you can resize the playlist. (it looks like it's responding to WM_WINDOWPOSCHANGED by changing its position, which doesn't seem like the best way to achieve whatever effect they're trying to achieve, but I could be wrong, especially considering that I have no clue.)

Anyway, the only thing that I have found to reliably contain Winamp is to use VWMHideTaskOnSwitch and the Classic skin. If VWMHideTaskEtc worked better, you could probably use the Modern skin, but alas it ain't so.

I'm not completely sure why *VWMFix doesn't work on the playlist window (it has an owner...), but I am completely tired.