There should be no interlaced (or combed) frames when previewing the script with VirtualDub, or in any intermediate lossless AVI you create. The framerate should be 23.976fps, and every frame should be progressive. I suspect you didn't use the "Force film" option in DGIndex when creating the d2v file.

To be sure it's your source and not the script, try a script with just the line:

Mpeg2Source("\path\filename.d2v")

and check for combed frames.

The final MPEG-2 encode for DVD will end up being 29.97fps with 3:2 pulldown. Depending on how you view/open this video, you may see some combed frames in this file.