"DivX and XviD both encode to MPEG-4 ASP which has a lot of shortcomings that manifestly result in the infamous sludgy green-purple banding artefacts of the time, no matter how good the chosen encoder is."
That's the fault of your decoder; I never had that problem playing DivX or XviD videos in VLC.
If anything it would be an issue with the encoder, but it's not. Both DivX and XviD are prime pieces of software but they live within unbridgeable limitations of the MPEG-4 ASP profile. This or that individual may not care about the outcome, and they may use various post-processing to smooth out the creases, but the limitations are all the same real and inherently visible.
It's just that you have different (or no) post-processing filters running between those two video players. MPEG cannot be arbitrarily decoded, only arbitrarily encoded.
That's the fault of your decoder; I never had that problem playing DivX or XviD videos in VLC.