Also remember to make sure your preview window is set to display anamorphic pixel width -- your renders will be fine without this, but it is annoying and misleading when you're editing if you can't "see" the true aspect ratio in the preview window. I forget the exact tool bar menu, but you simply right-click on the preview video window, and choose something like "emulate playback device aspect ratio". It doesn't actually say that, but if you right-click the preview window, it'll be obvious which item to select.

Hello guys, I did some testing and it seems to be working. It's a combination of both your suggestions that does the trick. To get a good preview in Vegas on the computerscreen, it's a matter of chosing 'emulate playback device aspect ratio' and also setting the 4:3 footage in a 16:9 widescreen project. If you forget this last step, it won't work.

Now there is only one question left: I use a firewirecard and a Canopus ADVC110 to have the realtime preview on a small but new 4:3 tv. The image on this tv looks vertically stretched, while when I switch back to the preview on the computerscreen there you'll see the right aspect ratio. Is there anything I do to get a proper preview on the 4:3 tv? (The tv doesn't have a manual 4:3/16:9 switch).

Unfortunately, no. The TV obviously doesn't have the capability to detect and display widescreen video correctly. So, there has to be some intelligent device between the PC and the TV that will do it. I don't believe the Canopus box does anything but convert between DV and analog.