Ok, am I right in thinking I should be working in Full Processing mode when processing my Lagarith RGB24 encoded files in VirtualDub? I gather that otherwise it shall be processing in YUV mode. Is that true?

Hmmmm. I don't know the answer for sure, but the documentation says:

1) Video filters in VirtualDub currently only run in 32-bit RGB, so the video frames are always converted to 32-bit RGB if video filters are applied. Note that in previous versions of VirtualDub, enabling full processing mode would always forced a conversion to 32-bit RGB. This is no longer the case — if no video filters are used, this conversion step is omitted and the video is directly converted to the output format as in Slow Recompress mode.

2) In Slow Recompress mode, video frames are converted from their current format to the output format specified in the 'Video color depth' dialog. If the formats are the same, no conversion takes place.

So ..... it will depend in part on what 'processing' you have in mind.

Not quite. Full Processing mode guarantees a conversion to RGB, while the others just use the existing color spaces, if possible. In your case, they are the same.

Thanks for the guidance, gentlemen.