Arnie.d said:

Colorspaces are filterspecific but virtualdub itself can process any colorspace.

It says here http://www.virtualdub.org/filtersdk.html that the filter interface is RGB.

I can open any avi and compress it with whatever codec and colorspace in virtualdub.

Yes, but if you use VirtualDub filters, there is a conversion to RGB, which was the point in my original post about not using VirtualDub for scaling (scaling is done via a filter).

Besides I don't use any filter in virtualdub, I only use avisynth and feed it to virtualdub.

That's what I'd do too. Most(all?) of AviSynth's filters support YUV.

I don't. Besides the GOUT looks like shit and needs much more work than just scaling. So why not in the process make it anamorphic.
Also, the PAL version were made from the same NTSC master so it had to be resized (and cropped) anyway. Another chance to make it anamorphic.

Good point about the PAL version - I'd have made it anamorphic too if all you had to work with was the NTSC master.