ckhouston, thanks for the suggestions, and I was happy to perform them for the reasons you cited. Here is my report, numbered to correspond with your instructions (1-3), plus some additional tests (4-5) of mine.
Terms: "Original" means file source was 39GB .mov file, 1920x1080. "Result" means VOB files viewed in VLC.
1. ConvertXtoDVD ver.5102: Source original, Best Quality checkbox unticked, VSO linear downscaling (default): The resulting color saturation and contrast result were slightly worse with BQ unticked.
2. Ver.5102: Source original, BQ checkbox ticked, Lanczos downscaling: Result was slightly sharper with Lanczos than with Linear.
3. Older ver.50045: Source had to change, BQ checkbox ticked, Lanczos downscaling: The older version of Convertx crashed immediately upon selecting the huge original source file. This behavior is obviously corrected in the newer version. So, In order to run test 3 with the older version of Convertx, I converted and downsized the original file from a 39GB .mov file to a 2GB .mp4 file using Handbrake, leaving the resolution at 1920x1080, and the Handbrake "high profile" defaults. Amazingly, there was no visible loss of color saturation, contrast, or sharpness in the Handbrake mp4. I then ran test 3 using the mp4 source, and the result was quite good! The color saturation and contrast of the Convertx result was essentially the same as the source however it lost some sharpness, which I would expect when downsizing resolution from 1920x1080 to 720x480 for DVD.
5. Ver.5102: Source mp4, BQ checkbox ticked, Lanczos downscaling: Back to the newer version to compare results to the old version using the mp4 source. Color saturation, contrast, and sharpness were essentially the same as with ver.50045, all other things equal.
Conclusion: The Handbrake downsizing step proved essential to preserving the quality of the original huge, high quality source, in both versions of Convertx. And it was required in the older version to allow processing. In order to achieve the best color and sharpness, tick "Use best video quality" and use Lanczos downscaling. As a side note, the newer version produces significantly less color saturation and contrast compared to the older version when Linear (default) downscaling is used in both. In that scenario, the newer version also produces less VOB data than the older version (1.8GB vs. 2.3GB in this case). Thanks.