To my knowledge, VirtualDub does not automatically use a built-in deinterlacer.

The proper framerate of PAL is 25fps (50 fields) whether it is interlaced or progressive. Since the source is PAL-Film (which is simply sped-up from 24fps to 25fps) there shouldn't be any combing artifacts to begin with. PAL-Film is progressive by nature.

The only time you should need to deinterlace PAL is if the source is PAL-Video (like a TV show) or if it is a botched film-to-PAL conversion by the studio.

In short, it sounds like what you are seeing and capturing is exactly as it should be. No deinterlacing needed.